• A novel Mg 2 InSm prismatic platelet can form in the aged Mg−In−Sm alloys. • The platelet has an aspect ratio of ∼25 and can keep a coherent relation with matrix. • The platelet is deemed as generalized β ″ with a (Mg 2 In)Sm-type D0 19 structure. Prismatic precipitate platelet is always purposefully designed in the microstructure of magnesium alloys due to its greater contribution to yield stress. In this study, with an introduction of In into Mg-Sm system, a category of novel { 10 1 ¯ 0 } α prismatic platelets has replaced thoroughly the traditional β ′ precipitate formed in magnesium rare earth (Mg-RE) alloys. Herein, the microstructural characteristics of platelet are investigated particularly by atomic scale scanning transmission electron microscopy. It is confirmed that the platelet has a Mg 2 InSm composition and can maintain a coherent relationship with α-Mg matrix. Importantly, on account of the similarities between In and Mg atoms, the Mg 2 InSm prismatic platelet could be structurally categorized as a generalized β ″ precipitate with a (Mg 2 In)Sm-type D0 19 structure when both In and Mg are regarded as an equivalent atom. Thus, the addition of In into Mg-Sm alloy induces the formation of β ″ precipitate. Furthermore, the formed β ″ prismatic platelets generally have a large average aspect ratio. The findings are of great significance to construct the effective precipitation strengthening phases and optimize the microstructure of Mg-based alloys. With an addition of In into Mg-Sm system, a type of novel { 10 1 ¯ 0 } α prismatic platelet has occurred and replaced the traditional β \prime precipitate. The prismatic platelet has a composition of Mg 2 InSm and can be categorized into a generalized β ″ phase with a (Mg 2 In)Sm-type D0 19 structure when Mg and In are deemed as a mixed atom.