Petit set down the first eight Colorado hitters he faced Thursday, setting the record by striking out Charlie Culberson and drawing a standing ovation at AT&T Park. That broke the mark held by Mark Buehrle with 45 in a row for the Chicago White Sox in 2009.
The streak ended when the next batter, Rockies pitcher Jordan Lyles, doubled to left field.
Petit's string covered eight games, six of them in relief.
The 29-year-old journeyman from Venezuela fell one out shy of a perfect game against Arizona last September. Petit made this start in place of struggling Tim Lincecum, who was recently sent to the bullpen.