How to Lock Android App's Orientation to Portrait in Phones and Landscape in Tablets?

You just have to define the property below inside the activity element in your AndroidManifest.xml file. It will restrict your orientation to portrait.

android:screenOrientation="portrait"

Example:

        <activity
            android:name="com.example.demo_spinner.MainActivity"
            android:label="@string/app_name"
            android:screenOrientation="portrait" >
        </activity>

if you want this to apply to the whole app define the property below inside the application tag like so:

        <application>
         android:screenOrientation="sensorPortrait"
        </application>

Additionaly, as per Eduard Luca's comment below, you can also use screenOrientation="sensorPortrait" if you want to enable rotation by 180 degrees.

You have to add the android:screenOrientation="portrait" directive in your AndroidManifest.xml. This is to be done in your <activity> tag.

In addition, the Android Developers guide states that :

[...] you should also explicitly declare that your application requires either portrait or landscape orientation with the element. For example, <uses-feature android:name="android.hardware.screen.portrait" />.

I can see you have accepted an answer which doesn't solve your problem entirely:

android:screenOrientation="portrait" 

This will force your app to be portrait on both phones and tablets.

You can have the app forced in the device's "preferred" orientation by using

android:screenOrientation="nosensor"

This will lead to forcing your app to portrait on most phones phones and landscape on tablets. There are many phones with keypads which were designed for landscape mode. Forcing your app to portrait can make it almost unusable on such devices. Android is recently migrating to other types of devices as well. It is best to just let the device choose the preferred orientation.