Kempny was set to become a restricted free agent July 1. The extension runs through the 2017-18 season.
The 26-year-old Kempny had two goals and six assists in 50 games in his first season with the Blackhawks after coming over from the KHL. He played for the Czech Republic in this year's world championship, finishing with two goals and an assist in eight games.
The Blackhawks, who were swept by Nashville in the first round of the playoffs, announced the deal Saturday.