ViiV Healthcare, GlaxoSmithKline's HIV business, will acquire two operations specialising in late stage treatments and early phase medicines.
It is the latest acquisition in a record year for pharmaceuticals.
Global deals have reached $677.5bn, according to Dealogic, up 64% on 2014.
Healthcare companies have been forced to buy or swap assets to bolster growing areas of their businesses and shore up their defences as generic versions of previously patented medicines are released in the market.
Sales of Advair, an asthma treatment and a bestseller for GSK, are expected to decline following the introduction of a generic version in the US.
In the first nine months of the year, GSK reported that sales of Advair fell 19% to £1.2bn, whereas sales of HIV treatments rose 56% to £1.6bn.
The company had considered spinning off its 80% stake in ViiV Healthcare through a flotation that could have valued the business at £18bn.
However, it decided to hang on to the operation in May.
GSK said it would make an upfront payment of $317m to buy Bristol-Myers Squibb's late stage HIV drugs, with a further $518m dependent on sales and royalties.
The assets include Fostemsavir, which treats HIV infection and is at the final phase three development stage. That means that it can seek regulatory approval, which is expected to be filed in 2018.
It will also pay an initial $33m to acquire pre-clinical and early stage research assets and could hand over $587m if certain development and sales goals are met.
Earlier this week, AstraZeneca said it would spend $4bn to buy a 55% stake in Acerta Pharma, a US-Dutch company which is developing a leukaemia treatment that is at the phase three stage. It has an option to buy the remaining 45% in the company.
AstraZeneca, which last year saw off a £69bn takeover bid from US rival Pfizer, also announced it would acquire a number of respiratory medicines from Japan's Takeda for $575m.
This year has been marked with major deals in the pharmaceuticals sector. Last month, Pfizer announced a $160bn merger with Botox-maker Allergen while in March, GSK said it would swap its cancer drugs portfolio with Novartis' vaccines business.
Shares in GSK fell 6.5p to £13.35.
GlaxoSmithKline has ended a frenetic week for the pharmaceutical sector with a $1.4bn (£939m) takeover of HIV drug development assets from Bristol-Myers Squibb.