Lithium orthosilicate is a compound with the chemical formula Li<sub>4</sub>SiO<sub>4</sub>. It is a white ceramic compound, which melts congruently at a temperature of .
Lithium orthosilicate is of primary interest towards carbon dioxide capture, as this compound reacts with CO<sub>2</sub> at elevated temperatures to form lithium carbonate, and has been implemented in limited scale in such applications. Further applications of Li<sub>4</sub>SiO<sub>4</sub> include solid electrolytes for lithium-ion batteries, specifically solid state batteries, and tritium breeding materials, as a component of the breeding blanket for planned fusion energy systems such as ITER.