We report the discovery of an eclipsing binary system with mass-ratio q ∼ 0.07. After identifying a periodic photometric signal received by WASP, we obtained CORALIE spectroscopic radial velocities and follow-up light curves with the Euler and TRAPPIST telescopes. From a joint fit of these data we determine that EBLM J0555-57 consists of a sun-like primary star that is eclipsed by a low-mass companion, on a weakly eccentric 7.8-day orbit. Using a mass estimate for the primary star derived from

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... tellar models, we determine a companion mass of 85 ± 4 M_ Jup (0.081M_) and a radius of 0.84^+0.14_-0.04 R_ Jup (0.084 R_) that is comparable to that of Saturn. EBLM J0555-57Ab has a surface gravity g_2 = 5.50^+0.03_-0.13 and is one of the densest non-stellar-remnant objects currently known. These measurements are consistent with models of low-mass stars.