We present UBVRI photometric measurements and spectroscopic observations of the double-lined eclipsing binary V821 Cas. The radial velocities were obtained by means of the cross-correlation technique. Simultaneous analyses of the multiband light curves and RVs give the absolute parameters for the stars as: M(1) = 2.05 +/- 0.07M(circle dot); M(2) = 1.63 +/- 0.06M(circle dot); R(1) = 2.31 +/- 0.03R(circle dot); R(2) = 1.39 +/- 0.02R(circle dot); T(eff1) = 9400 +/- 400 K and T(eff2) = 8 600 +/- 400 K. An analysis of the O-C residuals yielded an apsidal motion in the binary at a rate of omega = 0 degrees.0149 +/- 0 degrees.0023 cycle(-1), corresponding to an apsidal period of U = 118 +/- 19 yr. Subtracting the relativistic contribution, we find that log k(2obs) = -2.590 which is in agreement with the value predicted by theoretical models. Comparison with current stellar evolution models gives an age of 5.6 x 10(8) yr for the system.