diff --git a/src/D2L.CodeStyle.Analyzers/Diagnostics.cs b/src/D2L.CodeStyle.Analyzers/Diagnostics.cs
index 263daaa9..6eaa8ed8 100644
--- a/src/D2L.CodeStyle.Analyzers/Diagnostics.cs
+++ b/src/D2L.CodeStyle.Analyzers/Diagnostics.cs
@@ -774,5 +774,14 @@ public static class Diagnostics {
 			defaultSeverity: DiagnosticSeverity.Error,
 			isEnabledByDefault: true
 		);
+
+		public static readonly DiagnosticDescriptor PrimaryClassConstructorIntroducesMutability = new DiagnosticDescriptor(
+			id: "D2L0105",
+			title: "Primary class constructors introduce mutability into classes via their parameters",
+			messageFormat: "Primary class constructors introduce mutability into classes via their parameters",
+			category: "Immutability",
+			defaultSeverity: DiagnosticSeverity.Error,
+			isEnabledByDefault: true
+		 );
 	}
 }
diff --git a/src/D2L.CodeStyle.Analyzers/Immutability/ImmutabilityAnalyzer.cs b/src/D2L.CodeStyle.Analyzers/Immutability/ImmutabilityAnalyzer.cs
index 9c10fd29..f0cbf3fc 100644
--- a/src/D2L.CodeStyle.Analyzers/Immutability/ImmutabilityAnalyzer.cs
+++ b/src/D2L.CodeStyle.Analyzers/Immutability/ImmutabilityAnalyzer.cs
@@ -32,7 +32,9 @@ public sealed partial class ImmutabilityAnalyzer : DiagnosticAnalyzer {
 
 			Diagnostics.MissingTransitiveImmutableAttribute,
 			Diagnostics.InconsistentMethodAttributeApplication,
-			Diagnostics.UnappliedConditionalImmutability
+			Diagnostics.UnappliedConditionalImmutability,
+
+			Diagnostics.PrimaryClassConstructorIntroducesMutability
 		);
 
 		private readonly ImmutableHashSet<string> m_additionalImmutableTypes;
diff --git a/src/D2L.CodeStyle.Analyzers/Immutability/ImmutableDefinitionChecker.cs b/src/D2L.CodeStyle.Analyzers/Immutability/ImmutableDefinitionChecker.cs
index 371c7da2..98a9c084 100644
--- a/src/D2L.CodeStyle.Analyzers/Immutability/ImmutableDefinitionChecker.cs
+++ b/src/D2L.CodeStyle.Analyzers/Immutability/ImmutableDefinitionChecker.cs
@@ -51,6 +51,10 @@ public bool CheckDeclaration( INamedTypeSymbol type ) {
 				.Where( m => !m.IsStatic );
 
 			if( type.TypeKind == TypeKind.Class ) {
+				if( HasPrimaryClassConstructorMutability( type ) ) {
+					result = false;
+				}
+
 				// Check that the base class is immutable for classes
 				var baseClassOk = m_context.IsImmutable(
 					new ImmutabilityQuery(
@@ -81,6 +85,35 @@ out var diag
 			return result;
 		}
 
+		public bool HasPrimaryClassConstructorMutability( INamedTypeSymbol @class ) {
+			foreach( var constructor in @class.InstanceConstructors ) {
+				foreach( var syntaxRef in constructor.DeclaringSyntaxReferences ) {
+					var syntax = syntaxRef.GetSyntax( m_cancellationToken );
+
+					// Primary constructors for classes introduce mutability.
+					if( syntax is not ClassDeclarationSyntax ctor ) {
+						continue;
+					}
+
+					// Unimportant edge case
+					if( ctor.ParameterList.Parameters.Count == 0 ) {
+						return false;
+					}
+
+					m_diagnosticSink(
+						Diagnostic.Create(
+							Diagnostics.PrimaryClassConstructorIntroducesMutability,
+							ctor.ParameterList.GetLocation()
+						)
+					);
+
+					return true;
+				}
+			}
+
+			return false;
+		}
+
 		/// <remarks>
 		/// Check that a member (e.g. field or property) always produces immutable
 		/// values.
diff --git a/tests/D2L.CodeStyle.Analyzers.Test/Specs/ImmutabilityAnalyzer.cs b/tests/D2L.CodeStyle.Analyzers.Test/Specs/ImmutabilityAnalyzer.cs
index c5b0089a..fbee59a8 100644
--- a/tests/D2L.CodeStyle.Analyzers.Test/Specs/ImmutabilityAnalyzer.cs
+++ b/tests/D2L.CodeStyle.Analyzers.Test/Specs/ImmutabilityAnalyzer.cs
@@ -1461,4 +1461,14 @@ static ClassWithCapturedMutability() {
 			SometimesBad2 = static () => 2;
         }
     }
+
+	// This one doesn't create a diagnostic because there are no parameters.
+	[Immutable]
+	class ClassWithTrivialPrimaryConstructor();
+
+	[Immutable]
+	class ClassWithPrimaryConstructor /* PrimaryClassConstructorIntroducesMutability */ ( int X ) /**/;
+
+	[Immutable]
+	record class RecordClassCanHavePrimaryConstructor(int X);
 }