diff --git a/.changeset/thirty-pets-fry.md b/.changeset/thirty-pets-fry.md
new file mode 100644
index 0000000000..2a588bb161
--- /dev/null
+++ b/.changeset/thirty-pets-fry.md
@@ -0,0 +1,8 @@
+---
+'@backstage/frontend-plugin-api': minor
+'@backstage/core-plugin-api': minor
+---
+
+**BREAKING PRODUCERS**: The `IconComponent` no longer accepts `fontSize="default"`. This has effectively been removed from Material-UI since its last two major versions, and has not worked properly for them in a long time.
+
+This change should not have an effect on neither users of MUI4 nor MUI5/6, since the updated interface should still let you send the respective `SvgIcon` types into interfaces where relevant (e.g. as app icons).
diff --git a/packages/core-plugin-api/report.api.md b/packages/core-plugin-api/report.api.md
index d412f59b2d..06333c1fb4 100644
--- a/packages/core-plugin-api/report.api.md
+++ b/packages/core-plugin-api/report.api.md
@@ -505,14 +505,9 @@ export const googleAuthApiRef: ApiRef<
 >;
 
 // @public
-export type IconComponent = ComponentType<
-  | {
-      fontSize?: 'large' | 'small' | 'default' | 'inherit';
-    }
-  | {
-      fontSize?: 'medium' | 'large' | 'small' | 'inherit';
-    }
->;
+export type IconComponent = ComponentType<{
+  fontSize?: 'medium' | 'large' | 'small' | 'inherit';
+}>;
 
 // @public
 export type IdentityApi = {
diff --git a/packages/core-plugin-api/src/icons/types.test.ts b/packages/core-plugin-api/src/icons/types.test.ts
new file mode 100644
index 0000000000..fd01e176c9
--- /dev/null
+++ b/packages/core-plugin-api/src/icons/types.test.ts
@@ -0,0 +1,73 @@
+/*
+ * Copyright 2024 The Backstage Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import * as React from 'react';
+import { type IconComponent } from './types';
+
+// Emulate the MUI4 icon type
+type Mui4Icon = (props: {
+  fontSize?: 'default' | 'inherit' | 'large' | 'medium' | 'small';
+  otherProp?: string;
+}) => JSX.Element;
+
+// Emulate the MUI5 icon type
+type Mui5Icon = (props: {
+  fontSize?: 'inherit' | 'large' | 'medium' | 'small';
+  otherProp?: string;
+}) => JSX.Element;
+
+// Emulate the MUI6 icon type
+interface OverridableComponent<P> {
+  <RootComponent extends React.ElementType>(
+    props: {
+      /**
+       * The component used for the root node.
+       * Either a string to use a HTML element or a component.
+       */
+      component: RootComponent;
+    } & { stuff: number },
+  ): React.JSX.Element | null;
+  (props: P): React.JSX.Element | null;
+}
+type Mui6Icon = OverridableComponent<{
+  fontSize?: 'inherit' | 'large' | 'medium' | 'small';
+  otherProp?: string;
+}>;
+
+type NotAnIcon1 = (props: {
+  fontSize?: 'foo';
+  otherProp?: string;
+}) => JSX.Element;
+
+type NotAnIcon2 = (props: {
+  fontSize?: number;
+  otherProp?: string;
+}) => JSX.Element;
+
+describe('IconComponent', () => {
+  // eslint-disable-next-line jest/expect-expect
+  it('should be a component type', () => {
+    // @ts-ignore
+    let icon: IconComponent;
+    icon = {} as Mui4Icon;
+    icon = {} as Mui5Icon;
+    icon = {} as Mui6Icon;
+    // @ts-expect-error
+    icon = {} as NotAnIcon1;
+    // @ts-expect-error
+    icon = {} as NotAnIcon2;
+  });
+});
diff --git a/packages/core-plugin-api/src/icons/types.ts b/packages/core-plugin-api/src/icons/types.ts
index 4d54629de2..2b00e1456f 100644
--- a/packages/core-plugin-api/src/icons/types.ts
+++ b/packages/core-plugin-api/src/icons/types.ts
@@ -22,7 +22,7 @@ import { ComponentType } from 'react';
  *
  * @remarks
  *
- * The type is based on SvgIcon from Material UI, but both do not what the plugin-api
+ * The type is based on SvgIcon from Material UI, but we do not want the plugin-api
  * package to have a dependency on Material UI, nor do we want the props to be as broad
  * as the SvgIconProps interface.
  *
@@ -32,14 +32,6 @@ import { ComponentType } from 'react';
  *
  * @public
  */
-
-export type IconComponent = ComponentType<
-  /* Material UI v4 */
-  | {
-      fontSize?: 'large' | 'small' | 'default' | 'inherit';
-    }
-  /* Material UI v5: https://mui.com/material-ui/migration/v5-component-changes/#icon */
-  | {
-      fontSize?: 'medium' | 'large' | 'small' | 'inherit';
-    }
->;
+export type IconComponent = ComponentType<{
+  fontSize?: 'medium' | 'large' | 'small' | 'inherit';
+}>;
diff --git a/packages/frontend-plugin-api/report.api.md b/packages/frontend-plugin-api/report.api.md
index b575a94837..5bc2a663d7 100644
--- a/packages/frontend-plugin-api/report.api.md
+++ b/packages/frontend-plugin-api/report.api.md
@@ -1285,14 +1285,9 @@ export const IconBundleBlueprint: ExtensionBlueprint<{
 }>;
 
 // @public
-export type IconComponent = ComponentType<
-  | {
-      fontSize?: 'large' | 'small' | 'default' | 'inherit';
-    }
-  | {
-      fontSize?: 'medium' | 'large' | 'small' | 'inherit';
-    }
->;
+export type IconComponent = ComponentType<{
+  fontSize?: 'medium' | 'large' | 'small' | 'inherit';
+}>;
 
 // @public
 export interface IconsApi {
diff --git a/packages/frontend-plugin-api/src/icons/types.test.ts b/packages/frontend-plugin-api/src/icons/types.test.ts
new file mode 100644
index 0000000000..fd01e176c9
--- /dev/null
+++ b/packages/frontend-plugin-api/src/icons/types.test.ts
@@ -0,0 +1,73 @@
+/*
+ * Copyright 2024 The Backstage Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import * as React from 'react';
+import { type IconComponent } from './types';
+
+// Emulate the MUI4 icon type
+type Mui4Icon = (props: {
+  fontSize?: 'default' | 'inherit' | 'large' | 'medium' | 'small';
+  otherProp?: string;
+}) => JSX.Element;
+
+// Emulate the MUI5 icon type
+type Mui5Icon = (props: {
+  fontSize?: 'inherit' | 'large' | 'medium' | 'small';
+  otherProp?: string;
+}) => JSX.Element;
+
+// Emulate the MUI6 icon type
+interface OverridableComponent<P> {
+  <RootComponent extends React.ElementType>(
+    props: {
+      /**
+       * The component used for the root node.
+       * Either a string to use a HTML element or a component.
+       */
+      component: RootComponent;
+    } & { stuff: number },
+  ): React.JSX.Element | null;
+  (props: P): React.JSX.Element | null;
+}
+type Mui6Icon = OverridableComponent<{
+  fontSize?: 'inherit' | 'large' | 'medium' | 'small';
+  otherProp?: string;
+}>;
+
+type NotAnIcon1 = (props: {
+  fontSize?: 'foo';
+  otherProp?: string;
+}) => JSX.Element;
+
+type NotAnIcon2 = (props: {
+  fontSize?: number;
+  otherProp?: string;
+}) => JSX.Element;
+
+describe('IconComponent', () => {
+  // eslint-disable-next-line jest/expect-expect
+  it('should be a component type', () => {
+    // @ts-ignore
+    let icon: IconComponent;
+    icon = {} as Mui4Icon;
+    icon = {} as Mui5Icon;
+    icon = {} as Mui6Icon;
+    // @ts-expect-error
+    icon = {} as NotAnIcon1;
+    // @ts-expect-error
+    icon = {} as NotAnIcon2;
+  });
+});
diff --git a/packages/frontend-plugin-api/src/icons/types.ts b/packages/frontend-plugin-api/src/icons/types.ts
index 4d54629de2..2b00e1456f 100644
--- a/packages/frontend-plugin-api/src/icons/types.ts
+++ b/packages/frontend-plugin-api/src/icons/types.ts
@@ -22,7 +22,7 @@ import { ComponentType } from 'react';
  *
  * @remarks
  *
- * The type is based on SvgIcon from Material UI, but both do not what the plugin-api
+ * The type is based on SvgIcon from Material UI, but we do not want the plugin-api
  * package to have a dependency on Material UI, nor do we want the props to be as broad
  * as the SvgIconProps interface.
  *
@@ -32,14 +32,6 @@ import { ComponentType } from 'react';
  *
  * @public
  */
-
-export type IconComponent = ComponentType<
-  /* Material UI v4 */
-  | {
-      fontSize?: 'large' | 'small' | 'default' | 'inherit';
-    }
-  /* Material UI v5: https://mui.com/material-ui/migration/v5-component-changes/#icon */
-  | {
-      fontSize?: 'medium' | 'large' | 'small' | 'inherit';
-    }
->;
+export type IconComponent = ComponentType<{
+  fontSize?: 'medium' | 'large' | 'small' | 'inherit';
+}>;