diff --git a/src/material-experimental/mdc-dialog/dialog-content-directives.ts b/src/material-experimental/mdc-dialog/dialog-content-directives.ts
index 6cd76b9e283c..38de7efaf900 100644
--- a/src/material-experimental/mdc-dialog/dialog-content-directives.ts
+++ b/src/material-experimental/mdc-dialog/dialog-content-directives.ts
@@ -148,7 +148,7 @@ export class MatDialogContent {}
   },
 })
 export class MatDialogActions {
-  @Input() align?: 'center' | 'end' = undefined;
+  @Input() align?: 'start' | 'center' | 'end' = 'start';
 }
 
 /**
diff --git a/src/material/dialog/dialog-content-directives.ts b/src/material/dialog/dialog-content-directives.ts
index 29f5689d363c..0751984eb13d 100644
--- a/src/material/dialog/dialog-content-directives.ts
+++ b/src/material/dialog/dialog-content-directives.ts
@@ -152,7 +152,7 @@ export class MatDialogContent {}
   },
 })
 export class MatDialogActions {
-  @Input() align?: 'center' | 'end' = undefined;
+  @Input() align?: 'start' | 'center' | 'end' = 'start';
 }
 
 /**