•Stranger Things follows a group of kids in a fictional 1980s Indiana, with lots of supernatural goings on.
•Netflix says it counts a view when someone has watched at least 70% of an episode or film.
More than 40 million households watched the show in its first four days, according to Netflix, with 18 million-plus already having finished all eight episodes.
Netflix only recently started sharing viewing figures.
Bird Box topped the company's first list in January.
Eighty million people watched the horror-thriller film over four weeks.
Umbrella Academy, about a dysfunctional superhero family, was listed as having 45m viewers within its first month - the most for a series in the first quarter of 2019.
Stranger Things 3 has almost achieved that figure in a tenth of the time.
The figures are for the number of households that have watched the series, which is different to the total number of people who will have actually seen it.
Netflix says it counts a view when someone has watched at least 70% of an episode or film.
When They See Us, Ava DuVernay's drama series about the wrongfully-convicted Central Park Five, was announced as Netflix's most-watched series in the US following its May release.
But Netflix measured that using a different method - saying it had been the most watched series every day since its premiere - which is different to the Stranger Things "households" number.
Stranger Things - which follows a group of kids in a fictional 1980s Indiana, with lots of supernatural goings on - has been one of Netflix's most critically and culturally-acclaimed original series since its 2016 release.
It's helped launch the careers of Millie Bobby Brown and Finn Wolfhard, and introduced a new audience to Winona Ryder.
With its numbers seemingly so high, and fans emotional over how the third series ended, don't be surprised if there's still a lot more to come from the town of Hawkins.