How to set custom font for alert dialog in android?

To do this you use alert builder to build your alert. You then get the TextView from this alert and then you set the typeface for the alert.

AlertDialog dialog = new AlertDialog.Builder(this).setMessage("Hello world").show();
TextView textView = (TextView) dialog.findViewById(android.R.id.message);
Typeface face=Typeface.createFromAsset(getAssets(),"fonts/FONT"); 
textView.setTypeface(face); 

If you are using Material Components, you can customize your diaog to nearly all of needs by declearing a style for it. For example, custom style I created for my dialog:

    <style name="ThemeOverlay.App.MaterialAlertDialog" parent="ThemeOverlay.MaterialComponents.MaterialAlertDialog">
        <item name="materialAlertDialogTitleTextStyle"><!--here goes your title text style --></item>
        <item name="materialAlertDialogBodyTextStyle"><!--here goes your message text style --></item>
        <item name="colorPrimary"><!--here goes your dialog primary color. e.g. button text color, etc.--></item>
        <item name="shapeAppearanceOverlay">@style/ShapeAppearance.App.SmallComponent</item> <!-- your custom shape appearance for your dialog. In my case, I am changing corner radius of dialog to rounded 20dp corners-->
        <item name="colorSurface">@color/white</item>
        <item name="buttonBarPositiveButtonStyle">@style/Widget.App.Button</item> <!-- your custom positive button style-->
        <item name="buttonBarNegativeButtonStyle">@style/Widget.App.Button</item> <!-- your custom negtive button style-->
    </style>

    <style name="ShapeAppearance.App.SmallComponent" parent="ShapeAppearance.MaterialComponents.SmallComponent">
        <item name="cornerFamily">rounded</item>
        <item name="cornerSize">20dp</item>
    </style>

    <style name="Widget.App.Button" parent="Widget.MaterialComponents.Button.TextButton.Dialog">
        <item name="shapeAppearance">@style/ShapeAppearance.App.SmallComponent</item>
        <item name="android:textAppearance">@style/Roboto.Bold.Small</item>
        <item name="android:textColor">@color/colorAccent</item>
        <item name="android:textAllCaps">true</item>
    </style>

Finally, when creating your dialog, do not forget set this style:

 MaterialAlertDialogBuilder(this, R.style.ThemeOverlay_App_MaterialAlertDialog)
            .setMessage("your message")
            .show()

I know this is an old question, but I leave this here for those still searching for a solution.

If you only want to change text format, you can just override alertDialogTheme attribute to change the theme for the AlertDialog.

Example, using the Application theme:

<style name="MyTheme" parent="Theme.AppCompat.Light.NoActionBar">
    <item name="colorPrimary">@color/colorPrimary</item>
    <item name="colorPrimaryDark">@color/colorPrimaryDark</item>
    <item name="colorAccent">@color/colorAccent</item>

    <!-- This will override the Alert Dialog theme -->
    <item name="alertDialogTheme">@style/MyAlertDialogTheme</item>
</style>

<style name="MyAlertDialogTheme" parent="@android:style/Theme.Material.Light.Dialog.Alert">
    <item name="android:textAppearanceSmall">@style/MyTextAppearanceSmall</item>
    <item name="android:textAppearanceMedium">@style/MyTextAppearanceMedium</item>
    <item name="android:textAppearanceLarge">@style/MyTextAppearanceLarge</item>
</style>

<style name="MyTextAppearance" parent="TextAppearance.AppCompat">
    <item name="android:fontFamily">@font/comic_sans</item>
</style>
(...)

If I'm not mistaken android:textAppearanceSmall is used for the message and the android:textAppearanceMedium for the title. But you can pick whatever you want and remove the rest.

Another option

Without overriding the alertDialogTheme, is by setting the style via the builder constructor. Example: AlertDialog.Builder(getActivity(), R.style.MyAlertDialogTheme)

The above answers didnt work for me.

I used the following approach

// Initializing the alertDialog
AlertDialog alertDialog = new AlertDialog.Builder(QuizActivity.this).create();
alertDialog.setTitle("Warning");
alertDialog.setMessage("Are you sure you want to exit?");
alertDialog.show(); // This should be called before looking up for elements


// Getting the view elements
TextView textView = (TextView) alertDialog.getWindow().findViewById(android.R.id.message);
TextView alertTitle = (TextView) alertDialog.getWindow().findViewById(R.id.alertTitle);
Button button1 = (Button) alertDialog.getWindow().findViewById(android.R.id.button1);
Button button2 = (Button) alertDialog.getWindow().findViewById(android.R.id.button2);

// Setting font
textView.setTypeface(FontHelper.getFont(Fonts.MULI_REGULAR));
alertTitle.setTypeface(FontHelper.getFont(Fonts.MULI_REGULAR));
button1.setTypeface(FontHelper.getFont(Fonts.MULI_BOLD));
button2.setTypeface(FontHelper.getFont(Fonts.MULI_BOLD));

Tested on 7.1.1

NOTE: Make sure you get the element after showing the dialog. Without this you will get NullPointerException