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In this letter, we present the design, fabrication, and characterization of a high-efficiency in-plane splitting coupler for planar photonic crystal self-collimation devices. This splitting coupler consists of multiple concatenated in-plane reflective lenses and is capable of efficiently splitting and coupling a wide input beam into multiple narrow self-collimated beams. As an example, a one-to-two splitting coupler is simulated using the two-dimensional finite-difference time-domain method with an effective index used to account for the third dimension. The simulation shows a total coupling efficiency of 86% and a splitting ratio of two channels is 0.51 to 0.49. This device is also experimentally validated and the measured splitting ratio is 0.57 to 0.43.