Chyna Thomas, 22, died at the scene of the accident on Interstate 5 in Federal Way, Wash.
Isaiah Thomas received the news after taking part in the Celtics' practice in the afternoon, ESPN reported.
The network cited a source as saying that Thomas is expected to play on Sunday when the team opens its Eastern Conference first-round series against the Chicago Bulls.
"We are terribly saddened by the tragic loss of Chyna Thomas," the Celtics said in a statement Saturday. "The thoughts and prayers of the entire Celtics organization are with Isaiah and his family."
According to The News Tribune (Tacoma, Wash.), Chyna Thomas' 1998 Toyota Camry traveled off the road around 5 a.m. and hit a barrier, which State Patrol spokesman Nick King said "vaulted the vehicle head-on" into a sign post.
Thomas was not wearing her seatbelt at the time of the accident and died at the scene, according to the State Patrol.