The X-ray source RX J2015.6 + 3711 was discovered by ROSAT  in 1996 and recently proposed to be a cataclysmic variable ( CV ) . Here we report on an XMM–Newton observation of RX J2015.6 + 3711 performed in 2014 , where we detected a coherent X-ray modulation at a period of 7196 \pm 11  s and discovered other significant ( > 6 \sigma ) small-amplitude periodicities which we interpret as the CV spin period and the sidebands of a possible \sim 12 hr periodicity , respectively . The 0.3–10 keV spectrum can be described by a power law ( \Gamma = 1.15 \pm 0.04 ) with a complex absorption pattern , a broad emission feature at 6.60 \pm 0.01  keV , and an unabsorbed flux of ( 3.16 \pm 0.05 ) \times 10 ^ { -12 } erg cm ^ { -2 } s ^ { -1 } . We observed a significant spectral variability along the spin phase , which can be ascribed mainly to changes in the density of a partial absorber and the power law normalization . Archival X-ray observations carried out by the Chandra  satellite , and two simultaneous X-ray and UV/optical pointings with Swift , revealed a gradual fading of the source in the soft X-rays over the last 13 years , and a rather stable X-ray–to–optical flux ratio ( F _ { \mathrm { X } } / F _ { \mathrm { V } } \approx 1.4 - 1.7 ) . Based on all these properties , we identify this source with a magnetic CV , most probably of the intermediate polar type . The 2 hr spin period makes RX J2015.6 + 3711 the second slowest rotator of the class , after RX J0524 + 4244 ( `` Paloma '' ; P _ { spin } \sim 2.3  hr ) . Although we can not unambiguously establish the true orbital period with these observations , RX J2015.6 + 3711 appears to be a key system in the evolution of magnetic CVs .