The diffraction of optical guided waves in thin films through an acousto‐optic effect is treated. The diffraction of the bounded optical waves in thin films is a rather complicated phenomenon as compared with that of the free waves in bulk media. An expression for the diffraction efficiency has been derived using coupled‐wave equations. Acousto‐optical figure of merit and the mode coupling factor applicable to the diffraction in thin films are defined. The TE0and TM0waves in an As2S3film deposited onto a LiNbO3substrate were diffracted by Rayleigh waves with 93 and 72% efficiency at electrical input powers of 150 and 50 mW, respectively. The present experiment revealed that approximately 5% of the total acoustic power penetrated into the film and took part in the diffraction.