We report a remarkable Brønsted acid effect in the epoxidation of olefins by nonheme manganese catalysts and aqueous hydrogen peroxide. More specifically, a mononuclear nonheme manganese complex bearing a tetradentate N4 ligand, Mn^II(Dbp-MCP)(OTf)₂ (Dbp-MCP = (1R,2R)-N,N′-dimethyl-N,N′-bis((R)-(3,5-di-tert-butyl-phenyl)-2-pyridinylmethyl)cyclohexane-1,2-diamine; OTf^– = CF₃SO₃^–), is a highly efficient catalyst in the epoxidation of olefins by aqueous H₂O₂ in the presence of H₂SO₄ (1–3 mol %). The yields of epoxide products as well as the chemo- and enantioselectivities increase dramatically in the presence of H₂SO₄; no formation of epoxides is observed in the absence of H₂SO₄. In addition, the product yields and enantioselectivities are dependent significantly on the manganese catalysts and Brønsted acids. The catalytic epoxidation of olefins by other oxidants, such as peracids, alkyl hydroperoxides, and iodosylbenzene, is also affected by the presence of H₂SO₄; product yields and enantioselectivities are high and similar irrespective of the oxidants in the presence of H₂SO₄, suggesting that a common epoxidizing intermediate is generated in the reactions of [Mn^II(Dbp-MCP)]²⁺ and the oxidants. Mechanistic studies, performed with ¹⁸O-labeled water (H₂¹⁸O) and cumyl hydroperoxide, reveal that a high-valent manganese-oxo species is formed as an epoxidizing intermediate via O–O bond heterolysis of Mn-OOH(R) species. The role of H₂SO₄ is proposed to facilitate the formation of a high-valent Mn-oxo species and to increase the oxidizing power and enantioselectivity of the Mn-oxo oxidant in olefin epoxidation reactions. Density functional theory (DFT) calculations support experimental results such as the formation of a Mn(V)-oxo species as an epoxidizing intermediate.