β 3 -adrenergic receptor (β 3 -AR) activation produces a negative inotropic effect in human ventricles. Here we explored the role of β 3 -AR in the human atrium. Unexpectedly, β 3 -AR activation increased human atrial tissue contractility and stimulated the L-type Ca 2+ channel current (I Ca,L ) in isolated human atrial myocytes (HAMs). Right atrial tissue specimens were obtained from 57 patients undergoing heart surgery for congenital defects, coronary artery diseases, valve replacement, or heart transplantation. The I Ca,L and isometric contraction were recorded using a whole-cell patch-clamp technique and a mechanoelectrical force transducer. Two selective β 3 -AR agonists, SR58611 and BRL37344, and a β 3 -AR partial agonist, CGP12177, stimulated I Ca,L in HAMs with nanomolar potency and a 60%-90% efficacy compared with isoprenaline. The β 3 -AR agonists also increased contractility but with a much lower efficacy (~10%) than isoprenaline. The β 3 -AR antagonist L-748,337, β 1 -/β 2 -AR antagonist nadolol, and β 1 -/β 2 -/β 3 -AR antagonist bupranolol were used to confirm the involvement of β 3 -ARs (and not β 1 -/β 2 -ARs) in these effects. The β 3 -AR effects involved the cAMP/PKA pathway, since the PKA inhibitor H89 blocked I Ca,L stimulation and the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX) strongly increased the positive inotropic effect. Therefore, unlike in ventricular tissue, β 3 -ARs are positively coupled to L-type Ca 2+ channels and contractility in human atrial tissues through a cAMP-dependent pathway.