A swirl mixer for mixing a reducing agent with exhaust gas in a selective catalytic reduction (SCR) aftertreatment system is described. The swirl mixer may comprise a base permitting a flow of the reducing agent and the exhaust gas therethrough, and three arrays of fins projecting from the base in a direction of flow of the exhaust gas. The three arrays of fins may be arranged in a triangular configuration about a center of the mixer to induce a swirl motion to the reducing agent and the exhaust gas flowing through the mixer. The fins in each of the arrays may be oriented in a common direction that is rotated by about 60° from the common direction of the fins in an adjacent array.