A mononuclear non-heme Mn(III)-aqua complex, [(dpaq)Mn-III(OH2)](2+) (1, dpaq = 2-[bis-(pyridin-2-ylmethyl)]amino-N-quinolin-8-yl-acetamidate), is capable of conducting hydrogen atom transfer (HAT) reactions much more efficiently than the corresponding Mn(III)-hydroxo complex, [(dpaq)Mn-III(OH)](+) (2); the high reactivity of 1 results from the positive one-electron reduction potential of 1 (E-red vs SCE = 1.03 V), compared to that of 2 (E-red vs SCE = -0.1 V). The HAT mechanism of 1 varies between electron transfer followed by proton transfer and one-step concerted proton-coupled electron transfer, depending on the one-electron oxidation potentials of substrates. To the best of our knowledge, this is the first example showing that metal(III)-aqua complex can be an effective H-atom abstraction reagent.