In this paper we present an adaptive controller for the boost– based power factor precompensator which guarantees fast regulation of the output voltage towards a desired constant value with a power factor close to unity. This twofold control objective is fulfilled even in the presence of harmonics on the voltage source and uncertainties on the system parameters and load. The key for the solution to this problem is to express the model in terms of the input current instead of the inductor current. The resulting controller is reduced, through transformations, to the cascade interconnection of two controllers, namely the inner and the outer control loop. It is shown that while the latter turns out to be a simple lead–lag plus integration, the former is composed mainly of second order filters tuned at the frequencies of the considered harmonics and with transfer functions that follow a well defined pattern. Simulations are provided to assess the performance of the proposed controller. Keywords— AC-DC power conversion, power supplies, reactive power, dissipative systems, adaptive control, nonlinear systems.