We report spectroscopic observations of the 2.63day, detached, F-type main-sequence eclipsing binary V2154 Cyg. We use our observations together with existing uvby photometric measurements to derive accurate absolute masses and radii for the stars that are good to better than 1.5%. We obtain masses of M1=1.269+/-0.017M_{sun} and M2=0.7542+/-0.0059M{sun}, radii of R1=1.477+/-0.012R{sun} and R2=0.7232+/-0.0091R{sun}_, and effective temperatures of 6770+/-150K and 5020+/-150K for the primary and secondary stars, respectively. Both components appear to have their rotations synchronized with the motion in the circular orbit. A comparison of the properties of the primary with current stellar evolution models gives good agreement for a metallicity of [Fe/H]=-0.17, which is consistent with photometric estimates, and an age of about 2.2Gyr. On the other hand, the K2 secondary is larger than predicted for its mass by about 4%. Similar discrepancies are known to exist for other cool stars, and are generally ascribed to stellar activity. The system is in fact an X-ray source, and we argue that the main site of the activity is the secondary star. Indirect estimates give a strength of about 1kG for the average surface magnetic field on that star. A previously known close visual companion to V2154 Cyg is shown to be physically bound, making the system a hierarchical triple.