Diablo 3: Reaper of Souls sales topped 2.7 million units in the expansion's first week of availability, according to Activision Blizzard.
The company said on Thursday that the data is based on internal company records and reports from key distribution partners.
Diablo 3's first expansion pack launched on PC and Mac on March 25, almost two years after the game released in May 2012.
Blizzard CEO Mike Morhaime said: "With Reaper of Souls, we wanted to emphasize all the things that Diablo gamers have always loved about the series, from the dark gothic setting to the piles and piles of epic loot.
"We're excited to see that players are enjoying the changes we've made to the game, and we look forward to achieving our Paragon levels alongside everyone else in the weeks and months ahead."
CVG's 7/10 Diablo 3 Reaper of Souls review concluded: "Offering rebalanced systems, increased level caps and a new class, Reaper of Souls is worth one last crusade if you can stomach the price."
In November, Blizzard said lifetime Diablo 3 sales across all platforms had reached 14 million units. It also revealed Diablo 3: Ultimate Evil Edition for PS4. The game will include both Diablo 3 and Reaper of Souls.
In late March 2014, Diablo 3 lead producer Alex Mayberry said the game is also in development for Xbox One. However, the company isn't yet ready to officially announce whether the title will be released on Microsoft's console or not.