We report the discovery of 44.7 ms pulsations from the X-ray source CXOU J181335.1-174957 using data obtained with the XMM-Newton Observatory. PSR J1813-1749 lies near the center of the young radio supernova remnant G12.82-0.02, which overlaps the compact TeV source HESS J1813-178. This rotation-powered pulsar is the second most energetic in the Galaxy, with a spin-down luminosity of Edot = (6.8 +/- 2.7)E37 erg/s. In the rotating dipole model, the surface dipole magnetic field strength is B_s = (2.7 +/- 0.6)E12 G and the spin-down age of 3.3-7.5 kyr, consistent with the location in the small, shell-type radio remnant. At an assumed distance of 4.7 kpc by association with an adjacent young stellar cluster, the efficiency of PSR J1813-1749 in converting spin-down luminosity to radiation is approx. 0.03% for its 2-10 keV flux, approx. 0.1% for its 20-100 keV INTEGRAL flux, and approx. 0.07% for the >200 GeV emission of HESS J1813-178, making it a likely power source for the latter. The nearby young stellar cluster is possibly the birthplace of the pulsar progenitor, as well as an additional source of seed photons for inverse Compton scattering to TeV energies. ; Comment: 5 pages, 3 figure, Latex, emulateapj style. To appear in the Astrophysical Journal Letters