The answer to this often debated question is yes. The pitcher is allowed to throw to an unoccupied base if a runner is making an attempt to reach that base. The pitcher does not have to step off or clear the runner to make this throw.
For example, there is a runner on second base. The runner on second base breaks for third and the pitcher realizes what is happening. The pitcher being in his stretch delivery, steps toward third with his stride foot, and throws to third to get the runner out.
The rule as stated in the rule book makes this play legal. The pitcher is entitled to make a throw to second or third base if the runner is attempting to advance. When the runner makes an attempt to steal a bag, he is making an effort to advance. The pitcher has a right to make a play on an advancing runner. Then the pitcher is making a legal play in throwing to the base in an attempt to make the play.
Rule 8.05 in the official baseball rule book is the rule applying in this play. The rule states:
8.05 If there is a runner, or runners, it is a balk when (d) The pitcher, while touching his plate, throws, or feints a throw to an unoccupied base, except for the purpose of making a play.
The rule clearly states that a pitcher has the right to make a play on an advancing runner.
There are two things that the pitcher must remember when making this play:
1. The pitcher must step directly toward a base before throwing to that base. The 45-degree rule applies as judged by the umpire.
2. The pitcher can not stop his pitch motion to the plate to try and make a play on the runner. Once a pitcher commits his motion toward the plate, the pitch must be thrown to the plate.
I hope this article was informative and interesting to you. I thank you for taking the time to read it. Have a great day, Nick