It earned $4.5m (£3.1m) in the US and $2.67m (£1.8m) overseas at the weekend.
Starring Chris Rock and Samuel L Jackson, the newest film in the series follows the police hunt for a copycat inspired by the legacy of the "jigsaw killer" who set traps for his victims.
Saw debuted in 2004, with seven annual releases, before a revival in 2017.
Created by James Wan and Leigh Whannell, who also feature as executive producers on Spiral, the series was named the most successful horror movie series in film history by Guinness World Records in 2010.
