diff --git a/src/cdk/dialog/dialog.md b/src/cdk/dialog/dialog.md
index cfdd2bedbf6f..a324589660ff 100644
--- a/src/cdk/dialog/dialog.md
+++ b/src/cdk/dialog/dialog.md
@@ -129,6 +129,18 @@ bootstrapApplication(MyApp, {
 });
 ```
 
+> **Note:** The value provided for `DEFAULT_DIALOG_CONFIG` **replaces** the built-in defaults
+> entirely rather than merging with them. For example, providing `{disableClose: true}` means that
+> all other defaults (such as `hasBackdrop`) will be `undefined`. If you only want to override
+> specific properties, spread the defaults first:
+>
+> ```ts
+> {provide: DEFAULT_DIALOG_CONFIG, useValue: {...new DialogConfig(), disableClose: true}}
+> ```
+>
+> When you call `dialog.open()` with a config, that config is merged on top of these defaults, so
+> per-dialog options always take precedence.
+
 ### Sharing data with the Dialog component.
 You can use the `data` option to pass information to the dialog component.
 
diff --git a/src/material/dialog/dialog.md b/src/material/dialog/dialog.md
index 43a4cde25a5f..1424d81c9270 100644
--- a/src/material/dialog/dialog.md
+++ b/src/material/dialog/dialog.md
@@ -53,6 +53,18 @@ bootstrapApplication(MyApp, {
 });
 ```
 
+> **Note:** The value provided for `MAT_DIALOG_DEFAULT_OPTIONS` **replaces** the built-in defaults
+> entirely rather than merging with them. For example, providing `{disableClose: true}` means that
+> all other defaults (such as `hasBackdrop`) will be `undefined`. If you only want to override
+> specific properties, spread the defaults first:
+>
+> ```ts
+> {provide: MAT_DIALOG_DEFAULT_OPTIONS, useValue: {...new MatDialogConfig(), disableClose: true}}
+> ```
+>
+> When you call `dialog.open()` with a config, that config is merged on top of these defaults, so
+> per-dialog options always take precedence.
+
 ### Sharing data with the Dialog component.
 If you want to share data with your dialog, you can use the `data`
 option to pass information to the dialog component.