  We aimed to identify the variable X-ray source 3XMM J000511.8+634018, which
was serendipitously discovered through routine inspections while the 3XMM
catalogue was compiled. We analysed the archival XMM-Newton observation of the
source, obtained BUSCA photometry in three colours, and performed optical
spectroscopy with the LBT. These data were supplemented by archival
observations from the Zwicky Transient Facility. Based on its optical and X-ray
properties, 3XMM J000511.8+634018 is classified as a magnetic cataclysmic
variable, or polar. The flux is modulated with a period of 2.22 hr (8009.1 +-
0.2 s), which we identify with the orbital period. The bright phases are highly
variable in X-ray luminosity from one cycle to the next. The source shows a
thermal plasma spectrum typical of polars without evidence of a luminous soft
blackbody-like component. It is non-eclipsing and displays one-pole accretion.
The X-ray and BUSCA light curves show a stream absorption dip, which suggests
an inclination 50degr < i < 75degr. The phasing of this feature, which occurs
at the end of the bright phase, requires a somewhat special accretion geometry
with a stream running far around the white dwarf before it is magnetically
channelled. The period of this polar falls within the period gap of the
cataclysmic variables (2.15-3.18 h), but appears to fall just below the minimum
period when only polars are considered.
