Ros Casares jumped out to an early lead and were never threatened by Galatasaray, winning 77-47 behind Amaya Valdemoro's 21 points.
In the EuroLeague Women Group A game, the Spanish team led by 15 after one quarter, 22 at the half, and eventually stretched the lead to 35 after three quarters of play, much to the pleasure of the home crowd in Valencia.
Sophia Young led a short-handed Galatasaray with 20 points and nine rebounds.
The Turkish team did not travel with center Yelena Leuchanka, who had a knee problem.
Galatasaray also announced on Monday they had terminated the contract of Jia Perkins after the American's request to return home because of a family matter.
The good news for Galatasaray is the forthcoming arrival of Tamika Catchings.
The USA international has signed and will be with the team when they played in the Eighth-Final Play-offs.
On Wednesday early on, Spain international Valdemoro set the tone for the game, leading Ros Casares with nine first-quarter points, including two three-pointers.
DeLisha Milton and Erika De Souza both finished with 14 points, while Becky Hammon chipped in with 11.
The Turkish team was hindered by 19 turnovers, and they also only had three free-throw attempts.
The win moved Ros Casares to 8-2 while Galatasaray fell to 4-6 on the season.
ISMA CANTÓ, CIUDAD ROS CASARES Coach:
"The key of the game was in our good start. From the first minute we competed with aggressiveness on both ends of the court. We could beat Galatasaray and we finished in the fourth position in the Qualifying Round. Right now we don't have a easy road to the Final Four because we must to beat Prague but always we will play the definitives games at home."
LAIA PALAU, CIUDAD ROS CASARES:
"We have have done our homework and we will play the important games of the Play-Offs at home."
ZAFER KALAYCIOGLU, GALATASARAY Coach:
"We had important absences for this game in Valencia. Ciudad Ros Casares is a very difficult team at home. They beat Ekaterinburg by 19 points in Valencia. They played very well on offense. I hope that this season they could arribe to the final of the EuroLeague and they could compite to win the championship."