On June 23, 2007, Romero signed with the Philadelphia Phillies. During that season, Romero's ERA fell to just over 1.00. Romero improved a weak Phillies' bullpen plagued by inconsistency and injuries, which posted a 3.17 ERA in the final stretch of September. He also played a crucial role in the Phillies' triumph on the last day of the season, when in a 6–1 victory over the Nationals that capped the team's comeback against the New York Mets, Romero pitched one inning and struck out two. Romero was named the Phillies' top reliever for the post-season. On November 10, 2007, Romero agreed to a multi-year deal to remain with the Phillies.