The 6-foot-10 power forward's one-year deal for the veteran's minimum of $1.35 million is signed, sealed and delivered, Mavs president of basketball operations Donnie Nelson said.
"It's a done deal," Nelson said.
Thomas, 33, played 18 games for the Mavericks last season before leaving the team to care for his wife, who was fighting a still-undisclosed illness. The 13-year veteran owns career averages of 11.5 points and 4.1 rebounds, plus a 3-point percentage of 36.9.
He will provide depth at power forward behind Dirk Nowitzki.
Thomas takes the Mavs' roster to 14, leaving one spot available.