We present deep V, R, and I broadband CCD images of the northern radio lobe associated with the radio galaxy 3C 390.3. Spatially resolved emission coincident with the brightest region of the northern lobe (hot spot B) is detected in all filters, with surface brightness V=24.6 mag arcsec ^{-2} and V-I color ~ 0.8. The new optical data, together with the available radio and X-ray data for the hot spot, are compatible with a straight synchrotron spectrum of power-law index ~-0.8 . The source of optical emission is extended, and its shape and size are in good agreement with those seen at radio wavelengths. The second brightest region in the northern lobe (head A) was not detected in any of the filters. The upper limit to its brightness is I~23 mag.