diff --git a/src/Components/Components/ref/Microsoft.AspNetCore.Components.csproj b/src/Components/Components/ref/Microsoft.AspNetCore.Components.csproj
index 145bf52102..3f0b80424f 100644
--- a/src/Components/Components/ref/Microsoft.AspNetCore.Components.csproj
+++ b/src/Components/Components/ref/Microsoft.AspNetCore.Components.csproj
@@ -7,11 +7,13 @@
     <Compile Include="Microsoft.AspNetCore.Components.netstandard2.0.cs" />
     <Reference Include="Microsoft.Extensions.Logging.Abstractions"  />
     <Reference Include="Microsoft.Extensions.DependencyInjection.Abstractions"  />
+    <Reference Include="Microsoft.Extensions.HashCodeCombiner.Sources"  />
     <Reference Include="System.Buffers"  />
   </ItemGroup>
 <ItemGroup Condition="'$(TargetFramework)' == 'netcoreapp3.0'">
     <Compile Include="Microsoft.AspNetCore.Components.netcoreapp3.0.cs" />
     <Reference Include="Microsoft.Extensions.Logging.Abstractions"  />
     <Reference Include="Microsoft.Extensions.DependencyInjection.Abstractions"  />
+    <Reference Include="Microsoft.Extensions.HashCodeCombiner.Sources"  />
   </ItemGroup>
 </Project>
diff --git a/src/Components/Components/ref/Microsoft.AspNetCore.Components.netcoreapp3.0.cs b/src/Components/Components/ref/Microsoft.AspNetCore.Components.netcoreapp3.0.cs
index ed79b9c2ee..72adf28aa7 100644
--- a/src/Components/Components/ref/Microsoft.AspNetCore.Components.netcoreapp3.0.cs
+++ b/src/Components/Components/ref/Microsoft.AspNetCore.Components.netcoreapp3.0.cs
@@ -532,6 +532,8 @@ namespace Microsoft.AspNetCore.Components.Routing
     {
         public Router() { }
         [Microsoft.AspNetCore.Components.ParameterAttribute]
+        public System.Collections.Generic.IEnumerable<System.Reflection.Assembly> AdditionalAssemblies { [System.Runtime.CompilerServices.CompilerGeneratedAttribute]get { throw null; } [System.Runtime.CompilerServices.CompilerGeneratedAttribute]set { } }
+        [Microsoft.AspNetCore.Components.ParameterAttribute]
         public System.Reflection.Assembly AppAssembly { [System.Runtime.CompilerServices.CompilerGeneratedAttribute]get { throw null; } [System.Runtime.CompilerServices.CompilerGeneratedAttribute]set { } }
         [Microsoft.AspNetCore.Components.ParameterAttribute]
         public Microsoft.AspNetCore.Components.RenderFragment<Microsoft.AspNetCore.Components.RouteData> Found { [System.Runtime.CompilerServices.CompilerGeneratedAttribute]get { throw null; } [System.Runtime.CompilerServices.CompilerGeneratedAttribute]set { } }
diff --git a/src/Components/Components/ref/Microsoft.AspNetCore.Components.netstandard2.0.cs b/src/Components/Components/ref/Microsoft.AspNetCore.Components.netstandard2.0.cs
index ed79b9c2ee..72adf28aa7 100644
--- a/src/Components/Components/ref/Microsoft.AspNetCore.Components.netstandard2.0.cs
+++ b/src/Components/Components/ref/Microsoft.AspNetCore.Components.netstandard2.0.cs
@@ -532,6 +532,8 @@ namespace Microsoft.AspNetCore.Components.Routing
     {
         public Router() { }
         [Microsoft.AspNetCore.Components.ParameterAttribute]
+        public System.Collections.Generic.IEnumerable<System.Reflection.Assembly> AdditionalAssemblies { [System.Runtime.CompilerServices.CompilerGeneratedAttribute]get { throw null; } [System.Runtime.CompilerServices.CompilerGeneratedAttribute]set { } }
+        [Microsoft.AspNetCore.Components.ParameterAttribute]
         public System.Reflection.Assembly AppAssembly { [System.Runtime.CompilerServices.CompilerGeneratedAttribute]get { throw null; } [System.Runtime.CompilerServices.CompilerGeneratedAttribute]set { } }
         [Microsoft.AspNetCore.Components.ParameterAttribute]
         public Microsoft.AspNetCore.Components.RenderFragment<Microsoft.AspNetCore.Components.RouteData> Found { [System.Runtime.CompilerServices.CompilerGeneratedAttribute]get { throw null; } [System.Runtime.CompilerServices.CompilerGeneratedAttribute]set { } }
diff --git a/src/Components/Components/src/Microsoft.AspNetCore.Components.csproj b/src/Components/Components/src/Microsoft.AspNetCore.Components.csproj
index e92ae0634c..a7ad3493c9 100644
--- a/src/Components/Components/src/Microsoft.AspNetCore.Components.csproj
+++ b/src/Components/Components/src/Microsoft.AspNetCore.Components.csproj
@@ -1,4 +1,4 @@
-<Project Sdk="Microsoft.NET.Sdk">
+<Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
     <TargetFrameworks>netstandard2.0;netcoreapp3.0</TargetFrameworks>
@@ -16,6 +16,7 @@
   <ItemGroup>
     <Reference Include="Microsoft.Extensions.Logging.Abstractions" />
     <Reference Include="Microsoft.Extensions.DependencyInjection.Abstractions" />
+    <Reference Include="Microsoft.Extensions.HashCodeCombiner.Sources" />
   </ItemGroup>
 
   <ItemGroup Condition="'$(TargetFramework)'=='netstandard2.0'">
diff --git a/src/Components/Components/src/Routing/RouteTableFactory.cs b/src/Components/Components/src/Routing/RouteTableFactory.cs
index e0601bc95a..fc148b708a 100644
--- a/src/Components/Components/src/Routing/RouteTableFactory.cs
+++ b/src/Components/Components/src/Routing/RouteTableFactory.cs
@@ -7,6 +7,7 @@ using System.Collections.Generic;
 using System.Linq;
 using System.Reflection;
 using Microsoft.AspNetCore.Components.Routing;
+using Microsoft.Extensions.Internal;
 
 namespace Microsoft.AspNetCore.Components
 {
@@ -15,20 +16,21 @@ namespace Microsoft.AspNetCore.Components
     /// </summary>
     internal static class RouteTableFactory
     {
-        private static readonly ConcurrentDictionary<Assembly, RouteTable> Cache =
-            new ConcurrentDictionary<Assembly, RouteTable>();
+        private static readonly ConcurrentDictionary<Key, RouteTable> Cache =
+            new ConcurrentDictionary<Key, RouteTable>();
         public static readonly IComparer<RouteEntry> RoutePrecedence = Comparer<RouteEntry>.Create(RouteComparison);
 
-        public static RouteTable Create(Assembly appAssembly)
+        public static RouteTable Create(IEnumerable<Assembly> assemblies)
         {
-            if (Cache.TryGetValue(appAssembly, out var resolvedComponents))
+            var key = new Key(assemblies.OrderBy(a => a.FullName).ToArray());
+            if (Cache.TryGetValue(key, out var resolvedComponents))
             {
                 return resolvedComponents;
             }
 
-            var componentTypes = appAssembly.ExportedTypes.Where(t => typeof(IComponent).IsAssignableFrom(t));
+            var componentTypes = key.Assemblies.SelectMany(a => a.ExportedTypes.Where(t => typeof(IComponent).IsAssignableFrom(t)));
             var routeTable = Create(componentTypes);
-            Cache.TryAdd(appAssembly, routeTable);
+            Cache.TryAdd(key, routeTable);
             return routeTable;
         }
 
@@ -160,5 +162,61 @@ namespace Microsoft.AspNetCore.Components
 ");
             }
         }
+
+        private readonly struct Key : IEquatable<Key>
+        {
+            public readonly Assembly[] Assemblies;
+
+            public Key(Assembly[] assemblies)
+            {
+                Assemblies = assemblies;
+            }
+
+            public override bool Equals(object obj)
+            {
+                return obj is Key other ? base.Equals(other) : false;
+            }
+
+            public bool Equals(Key other)
+            {
+                if (Assemblies == null && other.Assemblies == null)
+                {
+                    return true;
+                }
+                else if (Assemblies == null ^ other.Assemblies == null)
+                {
+                    return false;
+                }
+                else if (Assemblies.Length != other.Assemblies.Length)
+                {
+                    return false;
+                }
+
+                for (var i = 0; i < Assemblies.Length; i++)
+                {
+                    if (!Assemblies[i].Equals(other.Assemblies[i]))
+                    {
+                        return false;
+                    }
+                }
+
+                return true;
+            }
+
+            public override int GetHashCode()
+            {
+                var hash = new HashCodeCombiner();
+
+                if (Assemblies != null)
+                {
+                    for (var i = 0; i < Assemblies.Length; i++)
+                    {
+                        hash.Add(Assemblies[i]);
+                    }
+                }
+
+                return hash;
+            }
+        }
     }
 }
diff --git a/src/Components/Components/src/Routing/Router.cs b/src/Components/Components/src/Routing/Router.cs
index ef34a47957..b93db4b6ac 100644
--- a/src/Components/Components/src/Routing/Router.cs
+++ b/src/Components/Components/src/Routing/Router.cs
@@ -4,6 +4,7 @@
 using System;
 using System.Collections.Generic;
 using System.Collections.ObjectModel;
+using System.Linq;
 using System.Reflection;
 using System.Threading.Tasks;
 using Microsoft.AspNetCore.Components.Rendering;
@@ -37,6 +38,12 @@ namespace Microsoft.AspNetCore.Components.Routing
         /// </summary>
         [Parameter] public Assembly AppAssembly { get; set; }
 
+        /// <summary>
+        /// Gets or sets a collection of additional assemblies that should be searched for components
+        /// that can match URIs.
+        /// </summary>
+        [Parameter] public IEnumerable<Assembly> AdditionalAssemblies { get; set; }
+
         /// <summary>
         /// Gets or sets the content to display when no match is found for the requested route.
         /// </summary>
@@ -64,6 +71,11 @@ namespace Microsoft.AspNetCore.Components.Routing
         {
             parameters.SetParameterProperties(this);
 
+            if (AppAssembly == null)
+            {
+                throw new InvalidOperationException($"The {nameof(Router)} component requires a value for the parameter {nameof(AppAssembly)}.");
+            }
+
             // Found content is mandatory, because even though we could use something like <RouteView ...> as a
             // reasonable default, if it's not declared explicitly in the template then people will have no way
             // to discover how to customize this (e.g., to add authorization).
@@ -79,7 +91,9 @@ namespace Microsoft.AspNetCore.Components.Routing
                 throw new InvalidOperationException($"The {nameof(Router)} component requires a value for the parameter {nameof(NotFound)}.");
             }
 
-            Routes = RouteTableFactory.Create(AppAssembly);
+
+            var assemblies = AdditionalAssemblies == null ? new[] { AppAssembly } : new[] { AppAssembly }.Concat(AdditionalAssemblies);
+            Routes = RouteTableFactory.Create(assemblies);
             Refresh(isNavigationIntercepted: false);
             return Task.CompletedTask;
         }
diff --git a/src/Components/Components/test/Routing/RouteTableFactoryTests.cs b/src/Components/Components/test/Routing/RouteTableFactoryTests.cs
index 8250fab16b..89bf2db1cc 100644
--- a/src/Components/Components/test/Routing/RouteTableFactoryTests.cs
+++ b/src/Components/Components/test/Routing/RouteTableFactoryTests.cs
@@ -11,6 +11,45 @@ namespace Microsoft.AspNetCore.Components.Test.Routing
 {
     public class RouteTableFactoryTests
     {
+        [Fact]
+        public void CanCacheRouteTable()
+        {
+            // Arrange
+            var routes1 = RouteTableFactory.Create(new[] { GetType().Assembly, });
+
+            // Act
+            var routes2 = RouteTableFactory.Create(new[] { GetType().Assembly, });
+
+            // Assert
+            Assert.Same(routes1, routes2);
+        }
+
+        [Fact]
+        public void CanCacheRouteTableWithDifferentAssembliesAndOrder()
+        {
+            // Arrange
+            var routes1 = RouteTableFactory.Create(new[] { typeof(object).Assembly, GetType().Assembly, });
+
+            // Act
+            var routes2 = RouteTableFactory.Create(new[] { GetType().Assembly, typeof(object).Assembly, });
+
+            // Assert
+            Assert.Same(routes1, routes2);
+        }
+
+        [Fact]
+        public void DoesNotCacheRouteTableForDifferentAssemblies()
+        {
+            // Arrange
+            var routes1 = RouteTableFactory.Create(new[] { GetType().Assembly, });
+
+            // Act
+            var routes2 = RouteTableFactory.Create(new[] { GetType().Assembly, typeof(object).Assembly, });
+
+            // Assert
+            Assert.NotSame(routes1, routes2);
+        }
+
         [Fact]
         public void CanDiscoverRoute()
         {
diff --git a/src/Components/test/E2ETest/Tests/RoutingTest.cs b/src/Components/test/E2ETest/Tests/RoutingTest.cs
index dd161bc7a3..4937fa9df9 100644
--- a/src/Components/test/E2ETest/Tests/RoutingTest.cs
+++ b/src/Components/test/E2ETest/Tests/RoutingTest.cs
@@ -418,6 +418,15 @@ namespace Microsoft.AspNetCore.Components.E2ETest.Tests
             Assert.Equal("Oops, that component wasn't found!", app.FindElement(By.Id("test-info")).Text);
         }
 
+        [Fact]
+        public void RoutingToComponentOutsideMainAppWorksWithAdditionalAssemblySpecified()
+        {
+            SetUrlViaPushState("/routeablecomponentfrompackage.html");
+
+            var app = MountTestComponent<TestRouterWithAdditionalAssembly>();
+            Assert.Contains("This component, including the CSS and image required to produce its", app.FindElement(By.CssSelector("div.special-style")).Text);
+        }
+
         [Fact]
         public void ResetsScrollPositionWhenPerformingInternalNavigation_LinkClick()
         {
diff --git a/src/Components/test/testassets/BasicTestApp/Index.razor b/src/Components/test/testassets/BasicTestApp/Index.razor
index 5acfe91fa0..f7751abd82 100644
--- a/src/Components/test/testassets/BasicTestApp/Index.razor
+++ b/src/Components/test/testassets/BasicTestApp/Index.razor
@@ -61,6 +61,7 @@
         <option value="BasicTestApp.ReorderingFocusComponent">Reordering focus retention</option>
         <option value="BasicTestApp.RouterTest.NavigationManagerComponent">NavigationManager Test</option>
         <option value="BasicTestApp.RouterTest.TestRouter">Router</option>
+        <option value="BasicTestApp.RouterTest.TestRouterWithAdditionalAssembly">Router with additional assembly</option>
         <option value="BasicTestApp.SvgComponent">SVG</option>
         <option value="BasicTestApp.SvgWithChildComponent">SVG with child component</option>
         <option value="BasicTestApp.TextOnlyComponent">Plain text</option>
diff --git a/src/Components/test/testassets/BasicTestApp/RouterTest/TestRouterWithAdditionalAssembly.razor b/src/Components/test/testassets/BasicTestApp/RouterTest/TestRouterWithAdditionalAssembly.razor
new file mode 100644
index 0000000000..c69f08f8c3
--- /dev/null
+++ b/src/Components/test/testassets/BasicTestApp/RouterTest/TestRouterWithAdditionalAssembly.razor
@@ -0,0 +1,11 @@
+@using Microsoft.AspNetCore.Components.Routing
+<Router AppAssembly="@typeof(BasicTestApp.Program).Assembly" AdditionalAssemblies="@(new[] { typeof(TestContentPackage.RouteableComponentFromPackage).Assembly, })">
+    <Found Context="routeData">
+        <RouteView RouteData="@routeData" />
+    </Found>
+    <NotFound>
+        <LayoutView Layout="@typeof(RouterTestLayout)">
+            <div id="test-info">Oops, that component wasn't found!</div>
+        </LayoutView>
+    </NotFound>
+</Router>