A stalagmite (UK /ˈstæləɡmaɪt/ or US /stəˈlæɡmaɪt/; from the Greek σταλαγμίτης - stalagmitês, from σταλαγμίας - stalagmias, "dropping, trickling") is a type of speleothem that rises from the floor of a limestone cave due to the dripping of mineralized solutions and the deposition of calcium carbonate. This stalagmite formation occurs only under certain pH conditions within the underground cavern. The corresponding formation on the ceiling of a cave is known as a stalactite. If these formations grow together, the result is known as a column.

Stalagmites should normally not be touched, since the rock buildup is formed by minerals precipitating out of the water solution onto the old surface; skin oils can alter the surface where the mineral water will cling, thus affecting the growth of the formation. Oils and dirt from human contact can also stain the formation and change its color permanently.

Similar structures can also form in lava tubes, associated with some types of lavacicles, although the mechanism of formation is very different. Stalactites and stalagmites can also form on concrete ceilings and floors, although they form much more rapidly there than in the natural cave environment.