We investigated the effect of hydrogen on the resistance to mechanically long fatigue crack growth in an equiatomic Fe–Cr–Ni–Mn–Co high-entropy alloy using compact tension (CT) tests at a frequency of 1 Hz and room temperature (20 °C). The CT test specimens were hydrogen charged using 100 MPa of hydrogen gas prior to testing. Fatigue crack growth progressed three times faster after exposure to hydrogen gas. The major change caused by hydrogen charging was the occurrence of intergranular crack growth. Striation features were observed in the intergranular fracture region, indicating that intergranular crack growth occurred via a plasticity-driven cycle-by-cycle mechanism. More specifically, we propose that the dislocation emission from the grain boundaries at a crack tip controls the intergranular fatigue crack growth. •Hydrogen accelerated fatigue crack growth of an FCC high-entropy alloy in CT tests.•Plasticity-assisted intergranular fatigue crack growth occurred.•Striation patterns appeared on the intergranular fracture surface.•The striation morphology was a peak-to-valley type.•Dislocation emission from grain boundaries at a crack tip is important.