With the continuous integration of new generation resources and evolving load profiles in modern power systems, the harmonic pollution in the electric power network is continuously on the rise. This level of waveform distortion in the electric power system and its impact on the operation of different components can no longer be neglected in high-voltage power systems. In this paper and beyond the state-of-the-art, an adaptive frequency-domain-based harmonic power flow (HPF) analytic is proposed for power systems with voltage source converter (VSC)-based high-voltage direct current (HVDC) transmission lines. The hybrid nonlinear model can be applied to both fundamental and harmonic power flow analyses at the point of common coupling. To demonstrate the effectiveness of the proposed HPF algorithm in hybrid AC/DC transmission systems, a modified pulse width modulation (PWM)-controlled bi-terminal VSC-HVDC transmission system based on the IEEE 30-bus test system is used as a testbed. Simulation results and numerical analysis verify the effective performance of the proposed solution as compared to the traditional approaches.