We report the discovery of a pulsar with period P = 136 ms and dispersion measure 308 cm-3 pc in a deep observation of the supernova remnant (SNR) G54.1+0.3 with the Arecibo radio telescope. Timing measurements of the new pulsar, J1930+1852, reveal a characteristic age of P/2 = 2900 yr and a spin-down luminosity of = 1.2 × 1037 ergs s-1. We have subsequently searched archival ASCA X-ray data of this SNR and detect pulsations with a consistent period. These findings ensure that PSR J1930+1852 is the pulsar that powers the "Crab-like" SNR G54.1+0.3. Together with existing Chandra observations of the SNR, we derive an X-ray pulsed fraction (2-10 keV) of 27%. We also find that the cooling efficiency of the pulsar wind nebula (PWN) is intermediate between those of the Vela and Crab PWNe: LX(2-10 keV) ~ 0.002. PSR J1930+1852 is a very weak radio source, with a period-averaged flux density at 1180 MHz of 60 μJy. For a distance of 5 kpc, its luminosity, ~1 mJy kpc2, is among the lowest for known young pulsars.