Context :

Aims : We investigate the physical nature of the X-ray emitting source 1RXS J165443.5 - 191620 through optical photometry and time-resolved spectroscopy .

Methods : Optical photometry is obtained from a variety of telescopes all over the world spanning \approx 27 days .
Additionally , time-resolved spectroscopy is obtained from the MDM observatory .

Results : The optical photometry clearly displays modulations consistent with those observed in magnetic cataclysmic variables : a low-frequency signal interpreted as the orbital period , a high-frequency signal interpreted as the white dwarf spin period , and an orbital sideband modulation .
Our findings and interpretations are further confirmed through optical , time-resolved , spectroscopy that displays H \alpha radial velocity shifts modulated on the binary orbital period .

Conclusions : We confirm the true nature of 1RXS J165443.5 - 191620 as an intermediate polar with a spin period of 546 seconds and an orbital period of 3.7 hours .
In particular , 1RXS J165443.5 - 191620 is part of a growing subset of intermediate polars , all displaying hard X-ray emission above 15keV , white dwarf spin periods below 30 minutes , and spin-to-orbital ratios below 0.1 .