Trench formation for device isolation on GaN light-emitting diode (LED) wafers via nanosecond ultraviolet laser micromachining is demonstrated. Trenches with smooth sidewalls and flat bottom surfaces are produced. Unlike wafer scribing with laser beams, the formation of trenches requires that the incident fluence is sufficient for laser ablation of GaN, yet low enough to prevent ablation of the sapphire substrate. Owing to the dissimilar ablation thresholds between GaN and sapphire, the etch process terminates automatically at the GaN/sapphire interface. The effect of the following parameters on the trench properties and quality has been investigated: focus offset, pulse energy, pulse repetition rate, scan speed, and the number of scan passes. It was found that optimal focus offset and pulse energy, a high pulse repetition rate, and single cycle of slow scanning are the key factors for obtaining a trench with tapered sidewall and smooth bottom surface, which is suitable for the laying of interconnects conformally across the trench for device interconnection. This technique has been successfully applied to the rapid prototyping of interconnected LED arrays on a single chip, where metal interconnects run continuously across the micromachined trenches to connect the individual LED devices.