A detailed kinematic analysis of ionized gas in the nearby irregular galaxy NGC 4449 is presented. Observations are conducted in the spectral lines of Halpha and [SII]. Our scanning Fabry--Perot interferometric observations are presented from both a global as well as a local perspective. We have analysed the global velocity field, the spatially extended diffuse gaseous component (DIG), the HII region populations, and, furthermore, have determined the rotation curve based on the heliocentric radial velocities of the global Halpha spatial distribution. Our results for NGC 4449 show that the optical velocity field has a decreasing value in radial velocity along the optical bar from NE to SW. The DIG component that permeates the entire galaxy was analysed (up to a limiting surface brightness of ~ 3.165x10^-5 ergs cm$^-2 s^-1 steradian^-1). We find that the diffuse gas component presents peculiar kinematical features such as abrupt velocity gradients and highly supersonic velocity dispersions (sigma~4 times the values of the nearest HII regions) but that its kinematical and dynamical influence is important on both global and local scales. The optical rotation curve of this nearby irregular shows that the NE sector rotates like a solid body (V_rot~40 km s^-1 at R=2 kpc). For the SW side, our results are not conclusive; the behavior of the gas at those locations is chaotic. We conclude that the origin of such complex kinematics and dynamics is undoubtedly related to the aftermath of an interaction experienced by this galaxy in the past (abridged).

AJ accepted, December issue, 33 pages, 13 figures