This paper presents the design of lumped quadrature power splitters (LQPSs) based on unit cells of right-handed (RH) and left-handed (LH) synthetic transmission lines (TLs). The LQPSs include a lumped Wilkinson splitter, with phase-adjusting RH/LH TLs at the outputs. Two topologies, considered to be advantageous with regards to size and electric characteristics, are studied in detail. For these two, closed-form design equations are derived and the performances are analyzed by circuit simulations. The theory and simulation results are experimentally validated by monolithic-microwave integrated-circuit prototypes designed for a center frequency of 2.5 GHz. Both prototypes have performance that agree well with theory and design simulations. Within the frequency range of 2–3 GHz, the maximum amplitude and phase errors are less than 0.3 dB and 3$^circ$, respectively. All reflections and the isolation are better than$-$10 dB. The effective areas of the two prototypes are 900$, times,$700$mu$m$^2$and 720$, times,$520$mu$m$^2$, respectively.