The 33-year-old has not been short of offers this summer, with the attacker widely expected to leave Betis following their relegation to the Segunda Division.
However, the former Spanish Under-21 international has now committed his future to the club.
"Our aim is set, hopefully we'll achieve it and my goals will help us gain promotion," Inside Spanish Football quotes Castro as saying.
"I feel we have a good enough team to do that, which is what everyone wants. I'm confident in the team, the way the manager trains us.
"He (Betis boss Julio Velazquez) wanted me to stay and has supported me 100% since he came here."
Castro has scored 71 league goals in 135 games for Betis.