Despite the secrecy around the film, a 1.2 gigabyte torrent of the movie had leaked ahead of its release.
The leak appears to have been filmed in a Chinese cinema and began to be shared on peer-to-peer piracy networks, according to TorrentFreak.
The illegally taped film, which found its way online between 4pm and 5pm local time on Wednesday, April 25, has since been uploaded onto English-language torrent networks.
The film was released in the UK on Wednesday, April 25, and the USA on Thursday, April 26 – with midnight screenings held around the world.
According to Torrentleak, the film is poor quality, partially rotated and is peppered with watermarks for a gambling site.
Audio for the stream, which is illegal to download and watch, is also described as “poor”.
Torrentleak has also detailed that a second, better quality copy of Endgame is circulating, sponsored by Russian betting company 1XBet.
Clips of the film can also be seen on Instagram, with one huge scene integral to the plot filmed from an English-speaking cinema.
Avengers: Endgame is expected to break box-office records, and is even tipped to close in on one of the highest-grossing film slots.
The 22nd film in the Marvel Cinematic Universe, Endgame ties up loose ends from Avengers: Infinity War – which saw many of fans favourite characters turned into dust at the hand of evil Titan Thanos.
Can you watch Avengers: Endgame online legally?
Avengers Endgame has only just been released in cinemas so cannot yet be watched online.
Any website purporting to have Endgame, the final installment in the Marvel series, available to stream is illegal.
But for those of you Marvel fans out there, you can watch other Marvel movies online from the comfort of your own home.
Ant-Man, and Guardians of the Galaxy are both on internet streaming giant Netflix.
While Avengers: Infinity War, the direct precursor to Endgame, is available to stream on Now TV.