We consider a control problem where the system is driven by a decoupled as well as a coupled forward-backward stochastic differential equation. We prove the existence of an optimal control in the class of relaxed controls, which are measure-valued processes, generalizing the usual strict controls. The proof is based on some tightness properties and weak convergence on the space D of cadlag functions, endowed with the Jakubowsky Stopology. Moreover, under some convexity assumptions, we show that the relaxed optimal control is realized by a strict control.