  We report on the discovery of pulsations at a period of ~47 s in the
persistent X-ray source 1RXS J225352.8+624354 (1RXS J2253) using five Chandra
observations performed in 2009. The signal was also detected in Swift and ROSAT
data, allowing us to infer over a 16-yr baseline an average, long-term period
increasing rate of ~17 ms per year and therefore to confirm the signal as the
spin period of an accreting, spinning-down neutron star. The pulse profile of
1RXS J2253 (~50-60% pulsed fraction) is complex and energy independent (within
the statistical uncertainties). The 1-10 keV Chandra spectra are well fit by an
absorbed power-law model with photon index ~1.4 and observed flux of (2-5)e-12
erg cm^-2 s^-1. The source was also detected by INTEGRAL in the 17-60 keV band
at a persistent flux of ~6e-12 erg cm^-2 s^-1, implying a spectral cut off
around 15 keV. We also carried out optical spectroscopic follow-up observations
of the 2MASS counterpart at the Nordic Optical Telescope. This made it possible
to first classify the companion of 1RXS J2253 as a B0-1III-Ve (most likely a
B1Ve) star at a distance of about 4-5 kpc (favouring an association with the
Perseus arm of the Galaxy). The latter finding implies an X-ray luminosity of
~3e34 erg s^-1, suggesting that 1RXS J2253 is a new member of the sub-class of
low-luminosity long-orbital-period persistent Be/X-ray pulsars in a wide and
circular orbit (such as X Persei).
