Yes. Negative answers are permitted on the digital SAT. Type the minus sign first, and note that it does not count against your digit allowance. You get 5 characters for a positive answer and 6 when a minus sign is included.
The character allowance is the detail worth remembering, because it is the one that silently truncates an answer. If a long decimal runs past the limit, the box simply stops accepting characters rather than warning you, so decide in advance whether you will enter a fraction or a rounded decimal.
The full picture, with the surrounding detail, is in SAT Grid-In Questions: The Format Rules Explained.