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Plasma waves generated in the wake of intense, relativistic laser or particlebeams can accelerate electron bunches to giga-electronvolt (GeV) energies incentimetre-scale distances. This allows the realization of compact acceleratorshaving emerging applications, ranging from modern light sources such as thefree-electron laser (FEL) to energy frontier lepton colliders. In a plasmawakefield accelerator, such multi-gigavolt-per-metre (GV m$^{-1}$) wakefieldscan accelerate witness electron bunches that are either externally injected orcaptured from the background plasma. Here we demonstrate optically triggeredinjection and acceleration of electron bunches, generated in a multi-componenthydrogen and helium plasma employing a spatially aligned and synchronized laserpulse. This ''plasma photocathode'' decouples injection from wake excitation byliberating tunnel-ionized helium electrons directly inside the plasma cavity,where these cold electrons are then rapidly boosted to relativistic velocities.The injection regime can be accessed via optical density down-ramp injection,is highly tunable and paves the way to generation of electron beams withunprecedented low transverse emittance, high current and 6D-brightness. Thisexperimental path opens numerous prospects for transformative plasma wakefieldaccelerator applications based on ultra-high brightness beams.