The lateral structure of gallium nitride (GaN) semiconductors enables monolithic integration of logic and power devices, which offers promise to miniaturize bulky converters into a compact package. However, the concentrated heat that arises from this dense integration can locally exceed 1 kW/cm2, which surpasses the limit of current thermal management technologies. In this article, we demonstrate the potential of integrating inchip microfluidic cooling directly on GaN power integrated circuits (ICs), together with additively manufactured packaging, to provide efficient thermal management, and achieve ultrahigh-power densities. A prototype power module and a 0.44 kW 48 V-24 V dc-dc converter were realized in a compact 32nd brick form factor to demonstrate its potential. Our results show a 14-fold reduction in thermal resistance and a fourfold increase in the total output power compared to heat-sink and fan cooling. An outstanding 78 kW/l was achieved, together with an increase in power conversion efficiency, surpassing 95%. By removing thermal limitations from power IC design, and enabling highly integrated topologies combined in a single liquidcooled chip, this work paves the way for more efficient and highly compact power conversion in the future to support the electrification of our society.