We present a demonstration of the electro‐optic tuning in a GaAs/AlGaAs quantum well vertical codirectional coupler filter. Tunable range over 2 nm was realized through the quantum‐confined Stark effect with a tuning rate of 0.28 nm/V2for transverse magnetic polarization. A filter bandwidth as narrow as 1.7 nm (full width at half‐maximum) at 830 nm with a coupling region length of 490 &mgr;m was obtained.