You can apply a centering to any View, including a Layout, by using the XML attribute android:layout_gravity". You probably want to give it the value "center".

You can find a reference of possible values for this option here: http://developer.android.com/reference/android/widget/LinearLayout.LayoutParams.html#attr_android:layout_gravity

Step #1: Wrap whatever it is you want centered on the screen in a full-screen RelativeLayout.

Step #2: Give that child view (the one, which you want centered inside the RelativeLayout) the android:layout_centerInParent="true" attribute.

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout
xmlns:android="http://schemas.android.com/apk/res/android"
android:id="@+id/relLayout1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="center">
<ProgressBar
android:id="@+id/ProgressBar01"
android:layout_centerInParent="true"
android:layout_width="wrap_content"
android:layout_gravity="center"
android:layout_height="wrap_content"></ProgressBar>
<TextView
android:layout_below="@id/ProgressBar01"
android:text="@string/please_wait_authenticating"
android:id="@+id/txtText"
android:paddingTop="30px"
android:layout_width="wrap_content"
android:layout_height="wrap_content"></TextView>
</RelativeLayout>

If you want to center one view, use this one. In this case TextView must be the lowermost view in your XML because it's layout_height is match_parent.

<TextView
android:id="@+id/tv_to_be_centered"
android:layout_height="match_parent"
android:layout_width="match_parent"
android:gravity="center"
android:text="Some text"
/>

I was able to center a view using

android:layout_centerHorizontal="true"

and

android:layout_centerVertical="true"

params.

Add android:layout_centerInParent="true" to element which you want to center in the RelativeLayout

It will work for that code sometimes need both properties

android:layout_gravity="center"
android:layout_centerHorizontal="true"

Updated answer: Constraint Layout

It seems that the trend in Android now is to use a Constraint layout. Although it is simple enough to center a view using a RelativeLayout (as other answers have shown), the ConstraintLayout is more powerful than the RelativeLayout for more complex layouts. So it is worth learning how do do now.

To center a view, just drag the handles to all four sides of the parent.

Use ConstraintLayout. Here is an example that will center the view according to the width and height of the parent screen:

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:layout_width="match_parent"
android:layout_height="match_parent">


<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="0dp"
android:layout_height="0dp"
android:background="#FF00FF"
android:orientation="vertical"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintHeight_percent=".6"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toRightOf="parent"
app:layout_constraintTop_toTopOf="parent"
app:layout_constraintWidth_percent=".4"></LinearLayout>
</android.support.constraint.ConstraintLayout>

You might need to change your gradle to get the latest version of ConstraintLayout:

dependencies {
...
implementation 'com.android.support.constraint:constraint-layout:1.1.3'
}

I use android:layout_centerInParent="true" and it worked

Use constraint layout. Please refer below code for the same.

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:layout_width="match_parent"
android:layout_height="match_parent"
>
<androidx.appcompat.widget.AppCompatTextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintBottom_toBottomOf="parent"
android:textSize="18sp"
android:textColor="@android:color/holo_orange_dark"
android:text="Centered TextView"
/>
</androidx.constraintlayout.widget.ConstraintLayout>