Is there a penalty for guessing wrong on the SAT or ACT?

Yes. Both the SAT and ACT have what is known as a "guessing penalty," which is a point deduction for each incorrect answer. The specific details of the guessing penalty vary between the two tests.

SAT

On the SAT, the guessing penalty is applied to the multiple-choice sections only (Reading, Writing and Language, and Math). For each question in these sections, you will lose 1/4 point for each incorrect answer, up to a maximum of 1 point per question.

ACT

On the ACT, the guessing penalty is applied to the multiple-choice sections only (English, Math, Reading, and Science). For each question in these sections, you will lose 1/4 point for each incorrect answer, up to a maximum of 2 points per question.

The guessing penalty is designed to discourage students from guessing on answers that they are not confident about. However, it is still possible to guess correctly on some questions, so it is not always a bad idea to guess if you are stuck.