ESPN's Adrian Wojnarowski first reported the deal Friday night.
This is the first in-season trade made by a Warriors team many expected to be active as the Feb. 6 deadline nears.
The 26-year-old big man is averaging 7.9 points, 6.2 rebounds and 1.5 assists in 22.9 minutes per night. The move comes just days after the Mavs announced center Dwight Powell ruptured his right Achilles tendon in Tuesday's loss to Los Angeles Clippers.
Cauley-Stein joined Golden State on a two-year, $4.46 million deal after spending the first four years of his career in Sacramento.
The big man's numbers are comparable to Powell's this year. Before his season-ending injury, Powell was averaging 9.4 points, 5.7 rebounds and 1.5 assists in 26.5 minutes per game.
This move is a massive win for Cauley-Stein, the sixth pick in 2015, who goes from a disastrous situation in Golden State to a playoff contender in Dallas. Breakout star Luka Doncic has the Mavericks in fifth place in the Western Conference.
Klay Thompson, a five-time All-Star, will likely miss the entire 2019-20 campaign for the Warriors while he recovers from a torn ACL, while Stephen Curry has been out since October with a broken hand. The two-time MVP is expected to return in March. Golden State has a league-worst record (10-36) and seems to have its eye on retooling the roster for the 2020-21 campaign.
Acquiring another draft selection in 2020 should help, whether the Warriors use the pick or package it in another deal. Both Glenn Robinson III and Alec Burks have been discussed in trade talks, according to Shams Charania of The Athletic.
This is likely just the start of a flurry of moves for a Warriors team that's ready to resume building its dynasty next season.