The 28-year-old, who scored one goal in three appearances for his country at Euro 2016, primarily lines up on the left flank but also has experience playing on the right and centrally.
"We are thrilled to be getting a high-level player in the prime of his career with top-level experience in the German Bundesliga and with the Hungarian national team," United general manager Dave Kasper said in a news release.
"Zoltan is a technically gifted player who can play multiple attacking positions, is excellent in possession, creates chances at high rates and who works on both sides of the ball for 90 minutes."
Stieber began his career with Aston Villa before spending most of the past decade in Germany, including stints with Mainz, Greuther Furth, Hamburg and Nurnberg. He joined Kaiserslautern last summer, making 19 appearances in the 2016-17 2. Bundesliga campaign.
United sits last in MLS at 5-14-4, with a league-worst 19 goals scored.