We discovered 2.8 s pulsations in the X-ray emission of the ultraluminous X-ray source (ULX) M51 ULX-7 within the UNSEeN project, which was designed to hunt for new pulsating ULXs (PULXs) with XMM-Newton. The pulse shape is sinusoidal and large variations of its amplitude were observed even within single exposures (pulsed fraction from less than 5% to 20%). M51 ULX-7 is a variable source, generally observed at an X-ray luminosity between $10^{39}$ and $10^{40}$ erg s$^{-1}$, located in the outskirts of the spiral galaxy M51a at a distance of 8.6 Mpc. According to our analysis, the X-ray pulsar orbits in a 2-d binary with a projected semi-major axis $a_mathrm{X} sin i simeq$ 28 lt-s. For a neutron star (NS) of 1.4 $M_{odot}$, this implies a lower limit on the companion mass of 8 $M_{odot}$, placing the system hosting M51 ULX-7 in the high-mass X-ray binary class. The barycentric pulse period decreased by $simeq$0.4 ms in the 31 d spanned by our May -- June 2018 observations, corresponding to a spin-up rate $dot{P} simeq -1.5times10^{-10}text{s s}^{-1}$. In an archival 2005 XMM-Newton exposure, we measured a spin period of $sim$3.3 s, indicating a secular spin-up of $dot{P}_{mathrm{sec}}simeq -10^{-9}text{ s s}^{-1}$, a value in the range of other known PULXs. Our findings suggest that the system consists of an OB giant and a moderately magnetic (dipole field component in the range $10^{12}$ G $lesssim B_{mathrm{dip}}lesssim 10^{13}$G) accreting NS with weakly beamed emission ($1/12lesssim blesssim1/4$).
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