How to debug apk signed for release?

Besides Manuel's way, you can still use the Manifest.

In Android Studio stable, you have to add the following 2 lines to application in the AndroidManifest file:

    android:debuggable="true"
    tools:ignore="HardcodedDebugMode"

The first one will enable debugging of signed APK, and the second one will prevent compile-time error.

After this, you can attach to the process via "Attach debugger to Android process" button.

I know this is old question, but future references. In Android Studio with Gradle:

buildTypes {
    release {
        debuggable true
        minifyEnabled true
        proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.txt'
    }
}

The line debuggable true was the trick for me.

Before Gradle 1.0 it was runProguard instead of minifyEnabled. Look at here.

Be sure that android:debuggable="true" is set in the application tag of your manifest file, and then:

1. Plug your phone into your computer and enable USB debugging on the phone
2. Open eclipse and a workspace containing the code for your app
3. In Eclipse, go to Window->Show View->Devices
4. Look at the Devices view which should now be visible, you should see your device listed
5. If your device isn't listed, you'll have to track down the ADB drivers for your phone before continuing
6. If you want to step through code, set a breakpoint somewhere in your app
7. Open the app on your phone
8. In the Devices view, expand the entry for your phone if it isn't already expanded, and look for your app's package name.
9. Click on the package name, and in the top right of the Devices view you should see a green bug along with a number of other small buttons. Click the green bug.
10. You should now be attached/debugging your app.