It will create additional school places, a sixth-form centre and brand new classrooms.
The three-storey building is now being used by pupils and teachers following an 18-month programme of work by Hillingdon Council, which started in May 2018.
The work means the school can offer an additional 186 places. It now has seven forms of entry and capacity for 1,302 pupils.
The borough’s £154m school building and expansion programme aims to meet an increasing demand for places.
Gareth Davies, head teacher at Ruislip High, said: “We have always had high aspirations for our sixth-form students and now we have a building that will provide high-quality education.”