We propose a novel all-optical feedforward automatic gain control (AGC) scheme for multicore erbium-doped fiber amplifiers (EDFAs). In this scheme, a semiconductor optical amplifier (SOA) is used in an AGC circuit. The first experimental demonstration of the dynamic gain control operation is reported. A maximum gain difference of ∼0.12 dB is successfully obtained using the control scheme. The proposed AGC circuit using an SOA achieves a significantly small response time constant of less than 5 µs, which is more than 20 times smaller than that of the circuit using an EDFA.