Nonlinear optical films on GaAs and other III-V (AlGaAs, InGaAs) semiconducting substrates have many potential applications in integrated optics. Until now, the growth of device quality films on GaAs was a major technological challenge. Using Pulsed Laser Deposition (PLD) epitaxial films of prototypical nonlinear KTa0.52Nb0.48O3 (KTN) were successfully deposited on single crystalline GaAs substrates for the first time. In order to alleviate chemical and structural incompatibilities between the GaAs substrate and KTN film at the growth temperature (∼750°C), a novel buffer layer scheme consisting of epitaxial MgO and SrTiO3 buffer layers and a Si3Ni4 encapsulation of the substrate was successfully developed. To obtain films with the required stoichiometry, a segmented target geometry was used consisting of KTN and KNO3 targets. The high crystalline quality of KTN films on GaAs was established by detailed 4-circle x-ray diffraction studies. Rutherford Backscattering studies confirmed the composition of the films. Optical measurements have been carried out on epitaxial KTN films on GaAs. Using an Ar+ laser (457.9 nm) operating in the TEM00 mode, we have observed sharp and distinguishable TE and TM propagating modes in KTN films. The refractive index (n0) of the film at 488 nm is 2.275 which is close to the bulk value of 2.35.