We report the discovery of 70.5 ms pulsations from the X-ray source AX J1838.0 - 0655 using the Rossi X-ray Timing Explorer ( RXTE ) . PSR J1838 - 0655 is a rotation-powered pulsar with spin-down luminosity \dot { E } = 5.5 \times 10 ^ { 36 } ergs s ^ { -1 } , characteristic age \tau _ { c } \equiv P / 2 \dot { P } = 23 kyr , and surface dipole magnetic field strength B _ { s } = 1.9 \times 10 ^ { 12 } G. It coincides with an unresolved INTEGRAL source and the extended TeV source HESS J1837 - 069 . At an assumed distance of 6.6 kpc by association with an adjacent massive star cluster , the efficiency of PSR J1838 - 0655 converting spin-down luminosity to radiation is 0.8 % for the 2 - 10 keV ASCA flux , 9 % for the 20 - 300 keV INTEGRAL flux and \sim 3 \% for the > 200 GeV emission of HESS J1837 - 069 , making it a plausible power source for the latter . A Chandra X-ray observation resolves AX J1838.0 - 0655 into a bright point source surrounded by a \approx 2 ^ { \prime } diameter , centrally peaked nebula . The spectra of the pulsar and nebula are each well fitted by power laws , with photon indices \Gamma = 0.5 ( 0.3 - 0.7 ) and \Gamma = 1.6 ( 1.1 - 2.0 ) , respectively . The 2 - 10 keV X-ray luminosities of the pulsar and nebula are L _ { PSR } = 4.6 \times 10 ^ { 34 } d _ { 6.6 } ^ { 2 } ergs s ^ { -1 } and L _ { PWN } = 5.2 \times 10 ^ { 33 } d _ { 6.6 } ^ { 2 } ergs s ^ { -1 } . A second X-ray source adjacent to the TeV emission , AX J1837.3 - 0652 , is resolved into an apparent pulsar/PWN ; it may also contribute to HESS J1837 - 069 . The star cluster RSGC1 may have given birth to one or both pulsars , while fueling TeV emission from the extended PWN with target photons for inverse Compton scattering .