The game sold 3.5 million copies in its first 24 hours of availability, according to Activision Blizzard's internal records and reports from key distribution partners.
The sales total doesn't include 1.2 million copies given to players who signed up for the World of Warcraft Annual Pass promotion.
6.3 million players had their hands on a copy of Diablo 3 at the end of its first week of availability, so it sold another 1.6 million copies in the six days following launch day.
Blizzard CEO Mike Morhaime said: "We're definitely thrilled that so many people around the world were excited to pick up their copy of Diablo 3 and jump in the moment it went live.
"We also regret that our preparations were not enough to ensure everyone had a seamless experience when they did so. I want to reaffirm our commitment to make sure the millions of Diablo 3 players out there have a great experience with the game moving forward, and I also want to thank them for their ongoing support."