The concept of (shear) Alfvèn wave resonant absorption for heating a plasma has been demonstrated on a theta pinch. The m = 1 mode is excited by means of a helical launching structure with a given wave length, at different frequencies. When the frequency is situated in the continuous Alfvèn spectrum, the kink energy is transmitted to the Alfvèn waves, where it is rapidly thermalized. The source term for the excitation is proportional to the density gradient. The measurement of the heating power is performed with a diamagnetic probe. We show that the probe signal can be treated as a thermodynamic variable which allows us to deduce an exact energy balance. The resonance curve of the heating power presents the peculiar shape predicted by theory. Strong power coupling exists between RF source and plasma, which follows from power balance considerations, allowing nearly all power delivered by the coils to be dissipated in the Alfvèn waves by resistivity and viscosity. The efficiency approaches unity, even in spite of the high compression ratio. The plasma motion is monitored by streak photography and shows nearly critical damping of the kink.