We analyse new integral-field spectroscopy of the inner region (central 2.5 kpc) of the spiral galaxy NGC 4321 to study the peculiar kinematics of this region. Fourier analysis of the velocity residuals obtained by subtracting an axisymmetric rotation model from the Hα velocity field indicates that the distortions are global features generated by an m = 2 perturbation of the gravitational potential which can be explained by the nuclear bar. This bar has been previously observed in the near-infrared but not in the optical continuum dominated by star formation. We detect the optical counterpart of this bar in the 2D distribution of the old stellar population (inferred from the equivalent width map of the stellar absorption lines). We apply the Tremaine-Weinberg method to the stellar velocity field to calculate the pattern speed of the inner bar, obtaining a value of Ωb = 160 +/- 70kms-1kpc-1. This value is considerably larger than the one obtained when a simple bar model is considered. However, the uncertainties in the pattern speed determination prevent us from giving support to alternative scenarios.