A novel scheme for the control of microwave signals carried at optical wavelengths by use of PbS colloidal quantum dots (QDs) embedded in PMMA waveguides is presented. When these structures are pumped at wavelengths where PbS has efficient absorption (980 or 1310 nm) a phase shift in a signal carried at 1550 nm is induced. Optimal conditions have been analyzed by studying the influence of the microwave signal and the waveguide structure. In a proof-of-concept experiment, a continuous phase shift up to 35º at 25 GHz has been demonstrated, with good thermal stability (<2º at 25 GHz) when the samples are heated 20ºC above room temperature. The potential benefits of the use of this active-waveguide technology in microwave photonics are due to the continuous scan of the phase delay, its high tuning speed, and its small size, which leads to the possibility of integration.