Hexamethylcyclotrisiloxane, also known as D3 and D3, is the organosilicon compound with the formula [(CH3)2SiO]3.

It is a colorless or white volatile solid.

The larger tetrameric and pentameric siloxanes, respectively octamethylcyclotetrasiloxane and decamethylcyclopentasiloxane, are of significant industrial interest,[1] whereas 1,000–10,000 tonnes per year of the trimer is manufactured and/or imported in the European Economic Area.

[2] Hexamethylcyclotrisiloxane adopts a planar structure and is considered strained.

[3][4] It reacts with organolithium reagents to give, after hydrolysis, dimethylsilanols: The LD50 for the related pentamer (D5) is >50 g/kg in rats.