On December 8, 2014, after becoming a free agent following the end of the 2014 season, Hammel agreed to terms on a two-year, $20 million deal to return to the Chicago Cubs. Hammel finished the 2015 season with a 10–7 record and a 3.74 ERA. He had the highest line drive percentage allowed (24.5%) of all major league pitchers. In game four of the 2015 National League Division Series vs. the St. Louis Cardinals, Hammel was the starting pitcher and pitched only 3 innings allowing three hits, two runs, one home run, and walked three batters with a no decision in the Cubs' 6–4 win over the Cardinals. Hammel also was the starting pitcher in game four of the 2015 National League Championship Series vs. the New York Mets. He only pitched innings, allowing four hits, five runs, two home runs, and walked two batters with the loss in the Cubs' 8–3 loss against the Mets.