  We report the discovery of a new AM Herculis binary (polar) as the optical
counterpart of the soft X-ray source RX J1724.0+4114 detected during the ROSAT
all-sky survey. The magnetic nature of this V~17 mag object is confirmed by
low-resolution spectroscopy showing strong Balmer and HeII emission lines
superimposed on a blue continuum which is deeply modulated by cyclotron humps.
The inferred magnetic field strength is 50+/-4 MG (or possibly even ~70 MG).
Photometric observations spanning ~3 years reveal a period of 119.9 min, right
below the period gap. The morphology of the optical and X-ray light curves
which do not show eclipses by the secondary star, suggests a self-eclipsing
geometry. We derive a lower limit of d~250 pc.
