Tobermorite is a calcium silicate hydrate mineral with chemical formula: Ca5Si6O16(OH)2·4H2O or Ca5Si6(O,OH)18·5H2O.

Two structural varieties are distinguished: tobermorite-11 Å and tobermorite-14 Å. Tobermorite occurs in hydrated cement paste and can be found in nature as an alteration mineral in metamorphosed limestone and in skarn.

Tobermorite was first described in 1880 for an occurrence in Scotland, on the Isle of Mull, around the locality of Tobermory.

[3][5] Aluminum-substituted tobermorite is understood to be a key ingredient responsible for the longevity of ancient undersea Roman concrete.

[6][7][8] Tobermorite is often used in thermodynamical calculations to represent the pole of the most evolved calcium silicate hydrate (C-S-H).