Context. M31N 2015-01a (or M31LRN 2015) is a red nova that erupted in January 2015 - the first event of this kind observed in M 31 since 1988. Very few similar events have been confirmed as of 2015. Most of them are considered to be products of stellar mergers. Aims: Results of an extensive optical monitoring of the transient in the period January-March 2015 are presented. Methods: Eight optical telescopes were used for imaging. Spectra were obtained on the Large Altazimuth Telescope (BTA), the Gran Telecsopio Canarias (GTC) and the Rozhen 2 m telescope. Results: We present a highly accurate 70 d light curve and astrometry with a 0.05''uncertainty. The colour indices reached a minimum of 2-3 d before peak brightness and rapidly increased afterwards. The spectral type changed from F5I to F0I in 6 d before the maximum and then to K3I in the next 30 d. The luminosity of the transient was estimated to be 8.7+3.3-2.2 × 105 L⊙ during the optical maximum. Conclusions: Both the photometric and the spectroscopic results confirm that the object is a red nova, similar to V838 Monocerotis. Table 4 is available in electronic form at http://www.aanda.org