We report the discovery of a low-mass (0.26 +/- 0.02, 0.15 +/- 0.01Msolar) pre-main-sequence (PMS) eclipsing binary (EB) with a 5.3d orbital period. JW 380 was detected as part of a high-cadence time-resolved photometric survey (the Monitor project) using the 2.5-m Isaac Newton Telescope and Wide Field Camera for a survey of a single field in the Orion nebula cluster (ONC) region in V and i bands. The star is assigned a 99 per cent membership probability from proper motion measurements, and radial velocity observations indicate a systemic velocity within 1σ of that of the ONC. Modelling of the combined light and radial velocity curves of the system gave stellar radii of 1.19+0.04-0.18 and 0.90+0.17-0.03Rsolar for the primary and the secondary, with a significant third light contribution which is also visible as a third peak in the cross-correlation functions used to derive radial velocities. The masses and radii appear to be consistent with stellar models for 2-3Myr age from several authors, within the present observational errors. These observations probe an important region of mass-radius parameter space, where there are currently only a handful of known PMS EB systems with precise measurements available in the literature.