We present an all-optical method to manipulate and fuse giant liposomes, typically 10 mum in diameter. Optical tweezers are used to trap two individual liposomes, which are then brought into contact. A single pulse of ultraviolet laser light focused on the adjoining membranes induces the fusion between the liposomes. As a consequence, the chemical reagents encapsulated in the two liposomes mix and a chemical reaction takes place.