Monolithic microwave integrated circuits (MMICs) with phase tuning are disclosed. A MMIC structure may include a MMIC amplifier with electrically coupled input and output lines. The MMIC structure may further include an adjustable cover over the MMIC amplifier that includes at least one portion that can be adjusted closer to or farther away from either the input or output lines. In this manner, a signal capacitance between the adjustable cover and the input or output lines is adjustable, and accordingly, a signal phase of the MMIC structure may be tuned. A spatial power-combining device may include a plurality of amplifier assemblies, wherein each amplifier assembly includes a MMIC amplifier with an adjustable cover. In this manner, the plurality of amplifier assemblies may be phase-tuned to a target value.