diff --git a/src/cdk/dialog/dialog-config.ts b/src/cdk/dialog/dialog-config.ts
index ff63a19068a4..584f3762991f 100644
--- a/src/cdk/dialog/dialog-config.ts
+++ b/src/cdk/dialog/dialog-config.ts
@@ -87,8 +87,12 @@ export class DialogConfig<D = unknown, R = unknown, C extends BasePortalOutlet =
   /** Dialog label applied via `aria-label` */
   ariaLabel?: string | null = null;
 
-  /** Whether this is a modal dialog. Used to set the `aria-modal` attribute. */
-  ariaModal?: boolean = true;
+  /**
+   * Whether this is a modal dialog. Used to set the `aria-modal` attribute. Off by default,
+   * because it can interfere with other overlay-based components (e.g. `mat-select`) and because
+   * it is redundant since the dialog marks all outside content as `aria-hidden` anyway.
+   */
+  ariaModal?: boolean = false;
 
   /**
    * Where the dialog should focus on open.
diff --git a/src/cdk/dialog/dialog.spec.ts b/src/cdk/dialog/dialog.spec.ts
index f224117dea08..bda16129413e 100644
--- a/src/cdk/dialog/dialog.spec.ts
+++ b/src/cdk/dialog/dialog.spec.ts
@@ -94,7 +94,18 @@ describe('Dialog', () => {
     viewContainerFixture.detectChanges();
     let dialogContainerElement = overlayContainerElement.querySelector('cdk-dialog-container')!;
     expect(dialogContainerElement.getAttribute('role')).toBe('dialog');
-    expect(dialogContainerElement.getAttribute('aria-modal')).toBe('true');
+    expect(dialogContainerElement.getAttribute('aria-modal')).toBe('false');
+  });
+
+  it('should be able to set aria-modal', () => {
+    dialog.open(PizzaMsg, {
+      viewContainerRef: testViewContainerRef,
+      ariaModal: true,
+    });
+    viewContainerFixture.detectChanges();
+
+    const container = overlayContainerElement.querySelector('cdk-dialog-container')!;
+    expect(container.getAttribute('aria-modal')).toBe('true');
   });
 
   it('should open a dialog with a template', () => {
@@ -117,7 +128,7 @@ describe('Dialog', () => {
 
     let dialogContainerElement = overlayContainerElement.querySelector('cdk-dialog-container')!;
     expect(dialogContainerElement.getAttribute('role')).toBe('dialog');
-    expect(dialogContainerElement.getAttribute('aria-modal')).toBe('true');
+    expect(dialogContainerElement.getAttribute('aria-modal')).toBe('false');
 
     dialogRef.close();
   });
diff --git a/src/material/bottom-sheet/bottom-sheet-config.ts b/src/material/bottom-sheet/bottom-sheet-config.ts
index ded882266ce6..6740e47bf4aa 100644
--- a/src/material/bottom-sheet/bottom-sheet-config.ts
+++ b/src/material/bottom-sheet/bottom-sheet-config.ts
@@ -44,8 +44,12 @@ export class MatBottomSheetConfig<D = any> {
   /** Aria label to assign to the bottom sheet element. */
   ariaLabel?: string | null = null;
 
-  /** Whether this is a modal bottom sheet. Used to set the `aria-modal` attribute. */
-  ariaModal?: boolean = true;
+  /**
+   * Whether this is a modal dialog. Used to set the `aria-modal` attribute. Off by default,
+   * because it can interfere with other overlay-based components (e.g. `mat-select`) and because
+   * it is redundant since the dialog marks all outside content as `aria-hidden` anyway.
+   */
+  ariaModal?: boolean = false;
 
   /**
    * Whether the bottom sheet should close when the user goes backwards/forwards in history.
diff --git a/src/material/bottom-sheet/bottom-sheet.spec.ts b/src/material/bottom-sheet/bottom-sheet.spec.ts
index 4f978f23767d..b632d66e7f99 100644
--- a/src/material/bottom-sheet/bottom-sheet.spec.ts
+++ b/src/material/bottom-sheet/bottom-sheet.spec.ts
@@ -153,7 +153,17 @@ describe('MatBottomSheet', () => {
 
     const containerElement = overlayContainerElement.querySelector('mat-bottom-sheet-container')!;
     expect(containerElement.getAttribute('role')).toBe('dialog');
-    expect(containerElement.getAttribute('aria-modal')).toBe('true');
+    expect(containerElement.getAttribute('aria-modal')).toBe('false');
+  });
+
+  it('should be able to set aria-modal', () => {
+    bottomSheet.open(PizzaMsg, {
+      ariaModal: true,
+    });
+    viewContainerFixture.detectChanges();
+
+    const container = overlayContainerElement.querySelector('mat-bottom-sheet-container')!;
+    expect(container.getAttribute('aria-modal')).toBe('true');
   });
 
   it('should close a bottom sheet via the escape key', fakeAsync(() => {
diff --git a/src/material/dialog/dialog-config.ts b/src/material/dialog/dialog-config.ts
index 3f50db03278e..66c106f15472 100644
--- a/src/material/dialog/dialog-config.ts
+++ b/src/material/dialog/dialog-config.ts
@@ -104,8 +104,12 @@ export class MatDialogConfig<D = any> {
   /** Aria label to assign to the dialog element. */
   ariaLabel?: string | null = null;
 
-  /** Whether this is a modal dialog. Used to set the `aria-modal` attribute. */
-  ariaModal?: boolean = true;
+  /**
+   * Whether this is a modal dialog. Used to set the `aria-modal` attribute. Off by default,
+   * because it can interfere with other overlay-based components (e.g. `mat-select`) and because
+   * it is redundant since the dialog marks all outside content as `aria-hidden` anyway.
+   */
+  ariaModal?: boolean = false;
 
   /**
    * Where the dialog should focus on open.
diff --git a/src/material/dialog/dialog.spec.ts b/src/material/dialog/dialog.spec.ts
index 1aa69b524ff6..b3752a42af52 100644
--- a/src/material/dialog/dialog.spec.ts
+++ b/src/material/dialog/dialog.spec.ts
@@ -112,7 +112,18 @@ describe('MatDialog', () => {
     viewContainerFixture.detectChanges();
     let dialogContainerElement = overlayContainerElement.querySelector('mat-dialog-container')!;
     expect(dialogContainerElement.getAttribute('role')).toBe('dialog');
-    expect(dialogContainerElement.getAttribute('aria-modal')).toBe('true');
+    expect(dialogContainerElement.getAttribute('aria-modal')).toBe('false');
+  });
+
+  it('should be able to set aria-modal', () => {
+    dialog.open(PizzaMsg, {
+      viewContainerRef: testViewContainerRef,
+      ariaModal: true,
+    });
+    viewContainerFixture.detectChanges();
+
+    const container = overlayContainerElement.querySelector('mat-dialog-container')!;
+    expect(container.getAttribute('aria-modal')).toBe('true');
   });
 
   it('should open a dialog with a template', () => {
@@ -134,7 +145,7 @@ describe('MatDialog', () => {
 
     let dialogContainerElement = overlayContainerElement.querySelector('mat-dialog-container')!;
     expect(dialogContainerElement.getAttribute('role')).toBe('dialog');
-    expect(dialogContainerElement.getAttribute('aria-modal')).toBe('true');
+    expect(dialogContainerElement.getAttribute('aria-modal')).toBe('false');
 
     dialogRef.close();
   });