[en] We report the discovery of the first eclipsing detached double white dwarf (WD) binary. In a pulsation search, the low-mass helium core WD NLTT 11748 was targeted for fast (∼1 minute) differential photometry with the Las Cumbres Observatory's Faulkes Telescope North. Rather than pulsations, we discovered ∼180 s 3%-6% dips in the photometry. Subsequent radial velocity measurements of the primary white dwarf from the Keck telescope found variations with a semi-amplitude K₁ = 271 ± 3 km s^-1 and confirmed the dips as eclipses caused by an orbiting WD with a mass M₂ = 0.648-0.771 M_sun for M₁ = 0.1-0.2 M_sun. We detect both the primary and secondary eclipses during the P_orb = 5.64 hr orbit and measure the secondary's brightness to be 3.5% ± 0.3% of the primary at SDSS-g'. Assuming that the secondary follows the mass-radius relation of a cold C/O WD and including the effects of microlensing in the binary, the primary eclipse yields a primary radius of R₁ = 0.043-0.039 R_sun for M₁ = 0.1-0.2 M_sun, consistent with the theoretically expected values for a helium core WD with a thick, stably burning hydrogen envelope. Though nearby (at ∼150 pc), the gravitational wave strain from NLTT 11748 is likely not adequate for direct detection by the Laser Interferometer Space Antenna. Future observational efforts will determine M₁, yielding accurate WD mass-radius measurement of both components, as well as a clearer indication of the binary's fate once contact is reached.