We report a GaN-based self-powered metal-semiconductor-metal (MSM)-type ultraviolet (UV) photodetector (PD) by employing a "lateral polarity structure (LPS)" grown on the sapphire substrate. An in-plane internal electric field and different Schottky barrier heights at a metal/semiconductor interface lead to efficient carrier separation and self-powered UV detection. A dark current of 6.8nA/cm2 and detectivity of 1.0×1012 Jones were obtained without applied bias. A high photo-to-dark current ratio of 1.2×104 and peak responsivity of 933.7 mA/W were achieved for the lateral polarity structure-photodetector (LPS-PD) under -10V. The enhanced performance of the LPS-PD was ascribed to the polarization-induced carrier separation as demonstrated by the lateral band diagram.