Altisite (IMA symbol: Ati) is an exceedingly rare alkaline titanium aluminosilicate chloride mineral with formula Na<sub>3</sub>K<sub>6</sub>Ti<sub>2</sub>Al<sub>2</sub>Si<sub>8</sub>O<sub>26</sub>Cl<sub>3</sub>, from alkaline pegmatites. It is named after its composition (ALuminium, TItanium, and SIlicon).
The mineral crystallizes in the monoclinic crystal system with space group C2/m.