We report the detection of a strong, organized magnetic field in the O9IV star HD 57682, using spectropolarimetric observations obtained with ESPaDOnS at the 3.6-m Canada–France–Hawaii Telescope within the context of the Magnetism in Massive Stars (MiMeS) Large Programme. From the fitting of our spectra using non-local thermodynamic equilibrium model atmospheres, we determined that HD 57682 is a 17+19−9 M⊙ star with a radius of 7.0+2.4−1.8 R⊙ and a relatively low mass-loss rate of 1.4+3.1−0.95× 10−9 M⊙ yr−1. The photospheric absorption lines are narrow, and we use the Fourier transform technique to infer v sin i= 15 ± 3 km s−1. This v sin i implies a maximum rotational period of 31.5 d, a value qualitatively consistent with the observed variability of the optical absorption and emission lines, as well as the Stokes V profiles and longitudinal field. Using a Bayesian analysis of the velocity-resolved Stokes V profiles to infer the magnetic field characteristics, we tentatively derive a dipole field strength of 1680+134−356 G. The derived field strength and wind characteristics imply a wind that is strongly confined by the magnetic field.