We present the design, fabrication and testing of a microfabricated planar reactor for the hydrogen evolution reaction (HER) using thin film Pt electrodes and polydimethylsiloxane (PDMS) fluidic chamber. The reactor is designed to separate gases by flow dynamics and reactor flow is analyzed by three-dimensional finite element analysis. The planar geometry is scalable, compact and stackable. Using KOH 28 wt% electrolyte, we have achieved a hydrogen generation density of 0.23 kg h(-1) m(-3) and an efficiency of 48% with a now rate of 10 ml min(-1) and cell voltage of 3 V. Published by Elsevier B.V.