We study the coherent and dissipative aspects of a driven spin-1 ?Bose-Einstein condensate (BEC) when the Zeeman energy is modulated ?around a static bias value. Resonances appearing when the bias energy ?matches an integer number of modulation quanta constitute the atomic ?counterpart of Shapiro resonances observed in microwave-driven ?superconducting Josephson junctions. The population dynamics near each ?resonance corresponds to slow and non-linear secular oscillations on ?top of a rapid “micro-motion”. At long times and in a narrow window of ?modulation frequencies around each resonance, we observe a relaxation ?to asymptotic states that are unstable without drive. These stationary ?states correspond to phase-locked solutions of the Josephson equations ?generalized to include dissipation, and are analogous to the ?stationary states of driven superconducting junctions. We find that ?dissipation is essential to understand this long-time behavior, and we ?propose a phenomenological model to explain quantitatively the ?experimental results. Finally, we demonstrate hysteresis in the ?asymptotic state of the driven spinor BEC when sweeping the modulation ?frequency across a Shapiro resonance.