In return, the Tigers sent pitcher Charlie Furbush, outfielder Casper Wells, third baseman Francisco Martinez and a player to be named later to the Mariners.
Detroit, which entered Saturday's games with a 2 1/2-game lead over Cleveland and three games over Chicago in the American League Central, bolstered both their starting and relief corps with the deal.
Fister, 27, compiled a 3.33 ERA over 21 starts for Seattle this season but only had a 3-12 record, thanks to lack of run support. The right-hander ranks last in the American League in run support average (1.97 runs).
Pauley, 28, racked up a 5-4 record with a 2.15 ERA in 39 relief appearances for Seattle this year.
The left-hander Furbush is 1-3 with a 3.62 ERA in 17 games, including two starts, for Detroit this season, while Wells, 26, appeared in 64 games with the Tigers this year before being optioned to Triple-A Toledo July 20.