The Turkey international averaged 2.4 points in nine minutes per game for Spartak as they won all but one of their EuroLeague Women group games.
She will bolster a Galatasaray side that has stormed into the Eighth-Finals of the EuroCup Women.
The Istanbul side swept past Municipal Targoviste 171-133 over two legs and will face K.V. Imperial AEL in the next round with the first game on January 29 in Cyprus.
Karsli, who agreed with Spartak to cancel her contract, has penned a one-and-a-half year deal with Galatasaray.