During a multiepoch radio continuum survey of premain sequence stars in the Rho Ophiuchi cloud made with the VLA, rapid radio variations were seen in the young star DoAr 21. Typically present at levels of 2-5 mJy with a flat spectral index, it rose on timescales of hours to 48 mJy at 5 GHz on February 18, 1983, during which the spectrum was steeply inverted. Interpretation in terms of mass loss, which is usually invoked to explain T Tauri radio emission, encounters difficulties. (Gyro)synchrotron radiation from energetic electrons in a magnetic loop several times larger than the star provides a better model. The event is very similar to radio flares seen on RS CVn stars. Along with an event seen in V410Tau, this is the first report of a microwave flare on a premain sequence star, and adds to the growing body of evidence that young stars exhibit very high levels of magnetically induced surface activity.