diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 66742c94f05..3d7e84321a5 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -1132,13 +1132,15 @@ import {
     walkUpBindingElementsAndPatterns,
     walkUpOuterExpressions,
     walkUpParenthesizedExpressions,
-    walkUpParenthesizedTypes,
-    walkUpParenthesizedTypesAndGetParentAndChild,
-    WhileStatement,
-    WideningContext,
-    WithStatement,
-    WriterContextOut,
-    YieldExpression,
+    walkUpParenthesizedTypes,
+    walkUpParenthesizedTypesAndGetParentAndChild,
+    Version,
+    versionMajorMinor,
+    WhileStatement,
+    WideningContext,
+    WithStatement,
+    WriterContextOut,
+    YieldExpression,
 } from "./_namespaces/ts.js";
 import * as moduleSpecifiers from "./_namespaces/ts.moduleSpecifiers.js";
 import * as performance from "./_namespaces/ts.performance.js";
@@ -47992,16 +47994,47 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
                 }
             }
 
-            if (isIdentifier(node.name)) {
-                checkCollisionsForDeclarationName(node, node.name);
-                if (!(node.flags & (NodeFlags.Namespace | NodeFlags.GlobalAugmentation))) {
-                    const sourceFile = getSourceFileOfNode(node);
-                    const pos = getNonModifierTokenPosOfNode(node);
-                    const span = getSpanOfTokenAtPosition(sourceFile, pos);
-                    suggestionDiagnostics.add(
-                        createFileDiagnostic(sourceFile, span.start, span.length, Diagnostics.A_namespace_declaration_should_not_be_declared_using_the_module_keyword_Please_use_the_namespace_keyword_instead),
-                    );
-                }
+            if (isIdentifier(node.name)) {
+                checkCollisionsForDeclarationName(node, node.name);
+                if (!(node.flags & (NodeFlags.Namespace | NodeFlags.GlobalAugmentation))) {
+                    const sourceFile = getSourceFileOfNode(node);
+                    const pos = getNonModifierTokenPosOfNode(node);
+                    const span = getSpanOfTokenAtPosition(sourceFile, pos);
+                    
+                    // Check if we should generate an error (TS 6.0+) or suggestion (older versions)
+                    const currentVersion = new Version(versionMajorMinor);
+                    const errorVersion = new Version("6.0");
+                    const shouldError = currentVersion.compareTo(errorVersion) >= Comparison.EqualTo;
+                    
+                    // Check if ignoreDeprecations should suppress this error
+                    let shouldSuppress = false;
+                    if (shouldError && compilerOptions.ignoreDeprecations) {
+                        // Only valid ignoreDeprecations values: "5.0" and "6.0"
+                        if (compilerOptions.ignoreDeprecations === "6.0") {
+                            shouldSuppress = true;
+                        }
+                    }
+                    
+                    if (shouldError && !shouldSuppress) {
+                        // In TypeScript 6.0+, this is an error unless suppressed by ignoreDeprecations
+                        const errorDiagnostic = createFileDiagnostic(
+                            sourceFile, 
+                            span.start, 
+                            span.length, 
+                            Diagnostics.The_module_keyword_is_not_allowed_for_namespace_declarations_Use_the_namespace_keyword_instead
+                        );
+                        diagnostics.add(errorDiagnostic);
+                    } else {
+                        // In older versions or when suppressed, keep as suggestion
+                        const suggestionDiagnostic = createFileDiagnostic(
+                            sourceFile, 
+                            span.start, 
+                            span.length, 
+                            Diagnostics.A_namespace_declaration_should_not_be_declared_using_the_module_keyword_Please_use_the_namespace_keyword_instead
+                        );
+                        suggestionDiagnostics.add(suggestionDiagnostic);
+                    }
+                }
             }
 
             checkExportsOnMergedDeclarations(node);
diff --git a/src/compiler/diagnosticMessages.json b/src/compiler/diagnosticMessages.json
index 22ac200a0dc..d44e112cbbc 100644
--- a/src/compiler/diagnosticMessages.json
+++ b/src/compiler/diagnosticMessages.json
@@ -1820,10 +1820,14 @@
         "category": "Error",
         "code": 1539
     },
-    "A 'namespace' declaration should not be declared using the 'module' keyword. Please use the 'namespace' keyword instead.": {
-        "category": "Suggestion",
-        "code": 1540,
-        "reportsDeprecated": true
+    "A 'namespace' declaration should not be declared using the 'module' keyword. Please use the 'namespace' keyword instead.": {
+        "category": "Suggestion",
+        "code": 1540,
+        "reportsDeprecated": true
+    },
+    "The 'module' keyword is not allowed for namespace declarations. Use the 'namespace' keyword instead.": {
+        "category": "Error",
+        "code": 1547
     },
     "Type-only import of an ECMAScript module from a CommonJS module must have a 'resolution-mode' attribute.": {
         "category": "Error",
diff --git a/tests/cases/fourslash/moduleDeclarationDeprecated_error1.ts b/tests/cases/fourslash/moduleDeclarationDeprecated_error1.ts
new file mode 100644
index 00000000000..9c6a6315d20
--- /dev/null
+++ b/tests/cases/fourslash/moduleDeclarationDeprecated_error1.ts
@@ -0,0 +1,27 @@
+///<reference path="fourslash.ts" />
+
+// @Filename: a.ts
+////[|module|] mod1 { export let x: number }
+////declare [|module|] mod2 { export let x: number }
+////export [|module|] mod3 { export let x: number }
+////export declare [|module|] mod4 { export let x: number }
+////namespace mod5 { export let x: number }
+////declare namespace mod6 { export let x: number }
+////mod1.x = 1;
+////mod2.x = 1;
+////mod5.x = 1;
+////mod6.x = 1;
+
+// @Filename: b.ts
+////declare module "global-ambient-module" {}
+
+goTo.file("a.ts")
+const errorDiagnostics = test.ranges().map(range => ({
+    code: 1547,
+    message: "The 'module' keyword is not allowed for namespace declarations. Use the 'namespace' keyword instead.",
+    range,
+}));
+verify.getSemanticDiagnostics(errorDiagnostics)
+
+goTo.file("b.ts")
+verify.getSemanticDiagnostics([])
\ No newline at end of file
diff --git a/tests/cases/fourslash/moduleDeclarationDeprecated_ignoreDeprecations.ts b/tests/cases/fourslash/moduleDeclarationDeprecated_ignoreDeprecations.ts
new file mode 100644
index 00000000000..6ca2f2d05e2
--- /dev/null
+++ b/tests/cases/fourslash/moduleDeclarationDeprecated_ignoreDeprecations.ts
@@ -0,0 +1,31 @@
+///<reference path="fourslash.ts" />
+
+// @ignoreDeprecations: 6.0
+// @Filename: a.ts
+////[|module|] mod1 { export let x: number }
+////declare [|module|] mod2 { export let x: number }
+////export [|module|] mod3 { export let x: number }
+////export declare [|module|] mod4 { export let x: number }
+////namespace mod5 { export let x: number }
+////declare namespace mod6 { export let x: number }
+////declare module "module-augmentation" {}
+////declare global {}
+////mod1.x = 1;
+////mod2.x = 1;
+////mod5.x = 1;
+////mod6.x = 1;
+
+// @Filename: b.ts
+////module "global-ambient-module" {}
+
+goTo.file("a.ts")
+const diagnostics = test.ranges().map(range => ({
+    code: 1540,
+    message: "A 'namespace' declaration should not be declared using the 'module' keyword. Please use the 'namespace' keyword instead.",
+    reportsDeprecated: true as true,
+    range,
+}));
+verify.getSuggestionDiagnostics(diagnostics)
+
+goTo.file("b.ts")
+verify.getSuggestionDiagnostics([])
\ No newline at end of file
diff --git a/tests/cases/fourslash/moduleDeclarationDeprecated_suggestion1.ts b/tests/cases/fourslash/moduleDeclarationDeprecated_suggestion1.ts
index 5554300a5a0..96f34910f7d 100644
--- a/tests/cases/fourslash/moduleDeclarationDeprecated_suggestion1.ts
+++ b/tests/cases/fourslash/moduleDeclarationDeprecated_suggestion1.ts
@@ -1,30 +1,31 @@
-///<reference path="fourslash.ts" />
-// @Filename: a.ts
-////[|module|] mod1 { export let x: number }
-////declare [|module|] mod2 { export let x: number }
-////export [|module|] mod3 { export let x: number }
-////export declare [|module|] mod4 { export let x: number }
-////namespace mod5 { export let x: number }
-////declare namespace mod6 { export let x: number }
-////declare module "module-augmentation" {}
-////declare global {}
-////mod1.x = 1;
-////mod2.x = 1;
-////mod5.x = 1;
-////mod6.x = 1;
-
-// @Filename: b.ts
-////module "global-ambient-module" {}
-
-goTo.file("a.ts")
-const diagnostics = test.ranges().map(range => ({
-    code: 1540,
-    message: "A 'namespace' declaration should not be declared using the 'module' keyword. Please use the 'namespace' keyword instead.",
-    reportsDeprecated: true,
-    range,
-}));
-verify.getSuggestionDiagnostics(diagnostics)
-
-goTo.file("b.ts")
+///<reference path="fourslash.ts" />
+// @ignoreDeprecations: 6.0
+// @Filename: a.ts
+////[|module|] mod1 { export let x: number }
+////declare [|module|] mod2 { export let x: number }
+////export [|module|] mod3 { export let x: number }
+////export declare [|module|] mod4 { export let x: number }
+////namespace mod5 { export let x: number }
+////declare namespace mod6 { export let x: number }
+////declare module "module-augmentation" {}
+////declare global {}
+////mod1.x = 1;
+////mod2.x = 1;
+////mod5.x = 1;
+////mod6.x = 1;
+
+// @Filename: b.ts
+////module "global-ambient-module" {}
+
+goTo.file("a.ts")
+const diagnostics = test.ranges().map(range => ({
+    code: 1540,
+    message: "A 'namespace' declaration should not be declared using the 'module' keyword. Please use the 'namespace' keyword instead.",
+    reportsDeprecated: true as true,
+    range,
+}));
+verify.getSuggestionDiagnostics(diagnostics)
+
+goTo.file("b.ts")
 verify.getSuggestionDiagnostics([])