Halak notched his seventh shutout of the year for Blues, who finished their 38-33-11 season with three wins in four games.
Alexander Steen and T.J. Oshie found the net for St. Louis.
The playoff-bound Predators, meanwhile, missed a chance to wrap up the fourth seed in the Western Conference and home ice for the first round.
Pekka Rinne turned aside 23 saves shots in Nashville's final regular-season contest.
The Predators (44-27-11) finished with 99 points.