We observed the field of GRB 240809A (Evans et al., GCN 37110; Want et al., GCN 37113; Dubai et al., GCN 37114; Jiang et al., GCN 37116) with the REM 60cm robotic telescope located at the ESO observatory of La Silla (Chile). The observations were carried in the g, r, i, z, J, H, K bands, starting on 2024 August 09 at 23:07:36 UT (i.e. 14.6 hours after the burst), and lasting for about 1 hour. From preliminary photometry, we detect the optical afterglow at a position consistent with the UVOT detection (Shilling & Evans, GCN 37123), at the following AB magnitude in the r band: r = 20.75 +- 0.20 (calibrated against PanSTARRS catalogue) at a mid-time of t-t0 ~ 15.1 hours after the GRB trigger.