With £5.18m including modest previews of £348,000, Alien: Covenant elbowed Guardians of the Galaxy Vol 2 aside after two weeks at the chart summit for the Marvel adventure.
The two films were the runaway winners at the weekend, between them responsible for more the two thirds of all UK box office receipts.
The first Guardians of the Galaxy film grossed £28.5m in summer 2014.
Previously, the biggest debut in the franchise was Alien: Resurrection (1997), which began with £2.67m in November 1997.