  We report on the redshift of the BL Lac object 3FGL J0909.0+2310 based on
observations obtained with the OSIRIS Multi Object Spectrograph (MOS) mounted
on the 10.4-m Gran Telescopio Canarias. A redshift of 0.432+/-0.002 was
obtained by the identification of three absorption features (CaII K&H and
G-band) detected in the spectrum of the BL Lac host galaxy. The closest object
to the BL Lac at an angular separation of 3.8" (~21 kpc at this distance) has a
similar redshift of 0.431+/-0.002. This companion galaxy could be the most
likely cause of the nuclear activity as postulated by studies based on more
extended data sets and cosmological models. MOS allows us to study the object's
neighbourhood within a field of view of approximately 7'x2' and we find two
small groups of galaxies at redshifts 0.28 and 0.39 which are probably not
related to the activity of 3FGL~J0909.0+2310.
