The 26-year-old Johns had a career-best 15 points (eight goals, seven assists) in 75 games last season, his third in Dallas. He was the prospect acquired in a deal that brought Patrick Sharp from Chicago in 2015.
The deal for Johns was announced Friday about an hour before the start of the NHL draft, which is being held in Dallas. Along with John Klingberg, Johns is part of a young core of defensemen the Stars hope can get the franchise back to the playoffs after a late-season collapse in 2017-18.
The 6-foot-4 native of Pennsylvania led the Stars with 155 blocked shots and 201 hits. Johns said he got his confidence back last season and "learned how to play the game at the NHL level and play at a consistent level."
General manager Jim Nill said Johns "plays a very fast, physical game and is just starting to hit the prime of his career."