How can I dismiss the on screen keyboard?

Note: This answer is outdated. See the answer for newer versions of Flutter.

You can dismiss the keyboard by taking away the focus of the TextFormField and giving it to an unused FocusNode:

FocusScope.of(context).requestFocus(FocusNode());

For Flutter version 2 or latest :

Since Flutter 2 with null safety this is the best way:

FocusManager.instance.primaryFocus?.unfocus();

Note: using old ways leads to some problems like keep rebuild states;

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For Flutter version < 2 :

As of Flutter v1.7.8+hotfix.2, the way to go is:

FocusScope.of(context).unfocus();

Comment on PR about that:

Now that #31909 (be75fb3) has landed, you should use FocusScope.of(context).unfocus() instead of FocusScope.of(context).requestFocus(FocusNode()), since FocusNodes are ChangeNotifiers, and should be disposed properly.

-> DO NOT use ̶r̶e̶q̶u̶e̶s̶t̶F̶o̶c̶u̶s̶(̶F̶o̶c̶u̶s̶N̶o̶d̶e̶(̶)̶ anymore.

 F̶o̶c̶u̶s̶S̶c̶o̶p̶e̶.̶o̶f̶(̶c̶o̶n̶t̶e̶x̶t̶)̶.̶r̶e̶q̶u̶e̶s̶t̶F̶o̶c̶u̶s̶(̶F̶o̶c̶u̶s̶N̶o̶d̶e̶(̶)̶)̶;̶

Read more about the FocusScope class in the flutter docs.