diff --git a/src/Microsoft.AspNetCore.Razor.Evolution/TagHelperMatchingConventions.cs b/src/Microsoft.AspNetCore.Razor.Evolution/TagHelperMatchingConventions.cs
index 9aa0ef8059..c1e01760ae 100644
--- a/src/Microsoft.AspNetCore.Razor.Evolution/TagHelperMatchingConventions.cs
+++ b/src/Microsoft.AspNetCore.Razor.Evolution/TagHelperMatchingConventions.cs
@@ -12,6 +12,8 @@ namespace Microsoft.AspNetCore.Razor.Evolution
     {
         public const string ElementCatchAllName = "*";
 
+        public const char ElementOptOutCharacter = '!';
+
         public static bool SatisfiesRule(
             string tagNameWithoutPrefix,
             string parentTagName,
@@ -66,6 +68,17 @@ namespace Microsoft.AspNetCore.Razor.Evolution
                 throw new ArgumentNullException(nameof(rule));
             }
 
+            if (string.IsNullOrEmpty(tagNameWithoutPrefix))
+            {
+                return false;
+            }
+
+            if (tagNameWithoutPrefix[0] == ElementOptOutCharacter)
+            {
+                // TagHelpers can never satisfy tag names that are prefixed with the opt-out character.
+                return false;
+            }
+
             if (rule.TagName != ElementCatchAllName &&
                 rule.TagName != null &&
                 !string.Equals(tagNameWithoutPrefix, rule.TagName, StringComparison.OrdinalIgnoreCase))
diff --git a/src/Microsoft.VisualStudio.LanguageServices.Razor/DefaultTagHelperFactsService.cs b/src/Microsoft.VisualStudio.LanguageServices.Razor/DefaultTagHelperFactsService.cs
index 555b46792c..30e2c381af 100644
--- a/src/Microsoft.VisualStudio.LanguageServices.Razor/DefaultTagHelperFactsService.cs
+++ b/src/Microsoft.VisualStudio.LanguageServices.Razor/DefaultTagHelperFactsService.cs
@@ -6,7 +6,6 @@ using Microsoft.AspNetCore.Razor.Evolution.Legacy;
 using System;
 using System.Collections.Generic;
 using System.ComponentModel.Composition;
-using System.Linq;
 
 namespace Microsoft.VisualStudio.LanguageServices.Razor
 {
@@ -48,8 +47,8 @@ namespace Microsoft.VisualStudio.LanguageServices.Razor
         }
 
         public override IEnumerable<BoundAttributeDescriptor> GetBoundTagHelperAttributes(
-            TagHelperDocumentContext documentContext, 
-            string attributeName, 
+            TagHelperDocumentContext documentContext,
+            string attributeName,
             TagHelperBinding binding)
         {
             if (documentContext == null)
diff --git a/test/Microsoft.VisualStudio.LanguageServices.Razor.Test/DefaultTagHelperFactsServiceTest.cs b/test/Microsoft.VisualStudio.LanguageServices.Razor.Test/DefaultTagHelperFactsServiceTest.cs
index bffe640ab8..90f0df6420 100644
--- a/test/Microsoft.VisualStudio.LanguageServices.Razor.Test/DefaultTagHelperFactsServiceTest.cs
+++ b/test/Microsoft.VisualStudio.LanguageServices.Razor.Test/DefaultTagHelperFactsServiceTest.cs
@@ -13,6 +13,26 @@ namespace Microsoft.VisualStudio.LanguageServices.Razor
         // Purposefully not thoroughly testing DefaultTagHelperFactsService.GetTagHelperBinding because it's a pass through
         // into TagHelperDescriptorProvider.GetTagHelperBinding.
 
+        [Fact]
+        public void GetTagHelperBinding_DoesNotAllowOptOutCharacterPrefix()
+        {
+            // Arrange
+            var documentDescriptors = new[]
+            {
+                ITagHelperDescriptorBuilder.Create("TestType", "TestAssembly")
+                    .TagMatchingRule(rule => rule.RequireTagName("*"))
+                    .Build()
+            };
+            var documentContext = TagHelperDocumentContext.Create(string.Empty, documentDescriptors);
+            var service = new DefaultTagHelperFactsService();
+
+            // Act
+            var binding = service.GetTagHelperBinding(documentContext, "!a", Enumerable.Empty<KeyValuePair<string, string>>(), parentTag: null);
+
+            // Assert
+            Assert.Null(binding);
+        }
+
         [Fact]
         public void GetTagHelperBinding_WorksAsExpected()
         {
@@ -133,6 +153,26 @@ namespace Microsoft.VisualStudio.LanguageServices.Razor
             Assert.Equal(expectedAttributeDescriptors, descriptors, BoundAttributeDescriptorComparer.CaseSensitive);
         }
 
+        [Fact]
+        public void GetTagHelpersGivenTag_DoesNotAllowOptOutCharacterPrefix()
+        {
+            // Arrange
+            var documentDescriptors = new[]
+            {
+                ITagHelperDescriptorBuilder.Create("TestType", "TestAssembly")
+                    .TagMatchingRule(rule => rule.RequireTagName("*"))
+                    .Build()
+            };
+            var documentContext = TagHelperDocumentContext.Create(string.Empty, documentDescriptors);
+            var service = new DefaultTagHelperFactsService();
+
+            // Act
+            var descriptors = service.GetTagHelpersGivenTag(documentContext, "!strong", parentTag: null);
+
+            // Assert
+            Assert.Empty(descriptors);
+        }
+
         [Fact]
         public void GetTagHelpersGivenTag_RequiresTagName()
         {