The planned 10-screen cinema will be run by global operator Showcase Cinema de Lux.
As well as airing the latest films, satellite broadcasts, music, arts and events, the new venue will feature a VIP gallery bar and lounge, plus a lobby bar and lounge to allow visitors to Buchanan Galleries to be entertained throughout the day and night.
The cinema will offer upgraded extra leg room seating in several screens as well as a large format screen with premium gallery seating.
It is hoped the new visitor attraction will open in 2017.
The extended Buchanan Galleries will deliver 1.2m sq ft of shopping, leisure and restaurant space and will be anchored by two major flagship stores - a new 150,000sq ft Marks & Spencer and a reconfigured 300,000sq ft John Lewis department store.
As well as the cinema, it will house more than 100 shops and 25 restaurants.
Showcase and Showcase Cinema de Lux operate more than 940 screens in the US, UK, Brazil and Argentina.
Buchanan Galleries developer Buchanan Partnership is a joint venture between Land Securities and Henderson Global Investors.
John Grimes, leasing director at Land Securities, said: "For Showcase Cinema de Lux, one of the leading international and UK cinema brands, to choose Buchanan Galleries for its next venue is a great vote of confidence in both the scheme and Glasgow.
"Glasgow is already ranked as the top retail destination in the UK outside London's West End and this deal will consolidate this position still further.
"We have found movie-goers enjoy the unparalleled level of entertainment experience that only Showcase Cinema de Lux can deliver and we fully expect the Buchanan Galleries to be one of the most successful cinemas in the UK."
Preparatory work on the new extension will begin later this year with construction work starting next summer.