Kasimovian Stage, third of four internationally defined stages of the Pennsylvanian Subsystem of the Carboniferous System, encompassing all rocks deposited during the Kasimovian Age (307 million to 303.7 million years ago). The name is taken from the Russian city of Kasimov, which lies east of Moscow in the Moscow Basin. The section is cyclic but consists mainly of limestones and dolomitic mudstones with intercalated siltstones.

The base of the stage is historically defined by the appearance of the advanced fusulinids (single-celled amoeba-like organisms with complex shells) Protriticites pseudomontiparus and Obsoletes obsoletus, with the ammonoid zone defined by the genus Parashumardites. This layer appears to correspond to the Westphalian-Stephanian Series boundary of western Europe and the Desmoinesian-Missourian boundary in the midcontinent of North America. The International Commission on Stratigraphy (ICS) has not yet established a Global Standard Section and Point for correlation at the base of this stage. The Kasimovian Stage is underlain by the Moscovian Stage and overlain by the Gzhelian Stage of the Pennsylvanian Subsystem.