We observed the optical afterglow of long bright GRB 240825A (Gupta et al., GCN 37274; Fermi GBM team, GCN 37273; Di Lalla et al., GCN 37288; Frederiks et al., GCN 37302) with the 3.6m Telescopio Nazionale Galileo (TNG) equipped with the DOLoRes camera in spectroscopic mode. Observations were carried out with the LR-B grism, covering the range 3500-8000 AA. The observations consisted of one spectrum carried out at a mean time of Aug 25.9562 UT (~7.4 hours after the burst). In the acquisition image we clearly detect the optical afterglow (Gupta et al., GCN 37274; Jiang et al. GCN 37275; Dutton et al. GCN 37276; Odeh et al. GCN 37277; Zhang et al. GCN 37278; Li et al. GCN 37280; Izzo & Malesani, GCN 37287; Lipunov et al. GCN 37289; Leonini et al. GCN 37291; Wu et al. GCN 37292; Brivio et al. GCN 32795; Odeh, GCN 37299) with a magnitude r~20, calibrated against the Pan-STARRS catalogue. The spectrum has a low SNR. From a preliminary reduction, we identify a single absorption feature consistent with Mg II at a redshift of z~0.66, in agreement with the results reported by Martin-Carrillo et al. (GCN 37293).