Santiago confirmed her departure to Gamasutra, explaining that she leaves on good terms with her colleagues but felt it was time to take on a new challenge.
"After doing three games, I think it was a really great opportunity for all of us to look at what we've learned and what I've taken from that experience, and go forth and take it into new arenas," she said. "Founding a company whose explicit goal was to do new types of game development really allowed me to try out new processes for game development, to try out new ideas.
"So much of my work at Thatgamecompany was really supporting Jenova's visions for the types of games he wanted to make, and I felt like I have done everything I needed to do there, and that he's in a great place now to go on and continue with some of the other people at Thatgamecompany, to take that to a whole new height."
While declining to go into detail on her future plans, Santiago hopes to "take what I've learned and use it to support more teams, more projects… and really, hopefully, help in accelerating the growth of videogames as a medium."
In a statement, Thatgamecompany – whose three-game deal with Sony ended with the release of Journey earlier this month – said: "While we want to continue the path of Thatgamecompany, Kellee has found a new direction in her career.
"Though our path in the future may be different, as TGC begins our next project, we wish Kellee a good journey and that our paths may cross again."