The ejecta composition is an open question in gamma-ray burst (GRB) physics(1). Some GRBs possess a quasi-thermal spectral component in the time-resolved spectral analysis(2), suggesting a hot fireball origin. Others show a featureless non-thermal spectrum known as the Band function(3-5), consistent with a synchrotron radiation origin(5,6) and suggesting that the jet is Poynting-flux dominated at the central engine and probably in the emission region as well(7,8). There are also bursts showing a sub-dominant thermal component and a dominant synchrotron component(9), suggesting a probable hybrid jet composition(10). Here, we report an extraordinarily bright GRB 160625B, simultaneously observed in gamma-ray and optical wavelengths, whose prompt emission consists of three isolated episodes separated by long quiescent intervals, with the durations of each sub-burst being approximately 0.8 s, 35 s and 212 s, respectively. Its high brightness (with isotropic peak luminosity L-p,L-iso approximate to 4 x 10(53) erg s(-1)) allows us to conduct detailed time-resolved spectral analysis in each episode, from precursor to main burst and to extended emission. The spectral properties of the first two sub-bursts are distinctly different, allowing us to observe the transition from thermal to non-thermal radiation between well-separated emission episodes within a single GRB. Such a transition is a clear indication of the change of jet composition from a fireball to a Poynting-flux-dominated jet.