The late goal kept Salt Lake three points in front of Portland and Colorado at the top of the Western Conference.
Grossman's goal came while Salt Lake was playing a man short, a situation brought about by a direct red card given Yordany Alvarez in the 79th minute. Soon after Alvarez was sent off, Kelif Alhassan scored for Portland to break a 2-2 tie and put the Timbers in excellent position to move into a tie for the conference lead.
Grossman, however, came through with the crucial goal that kept Salt Lake from losing two in a row and four out of six.
Salt Lake took a 2-1 lead in the first half, but Portland tied it in the 57th minute on a penalty kick from Diego Valeri. Javier Morales was called for a handball in the penalty area that led to Valeri's goal.