StarCraft 2: Heart of the Swarm sales reached approximately 1.1 million copies worldwide during the expansion's first two days of availability, according to Blizzard.
The studio said the figure includes both retail and digital sales.
Over 1.1 million viewers also tuned in to watch the company's 21-hour, global broadcast of launch events and community celebrations, according to Twitch.
Blizzard CEO Mike Morhaime said: "We want to thank the global StarCraft community for the incredible passion and support they've shown for Heart of the Swarm.
"In addition to a brand-new single-player campaign, we added a ton of enhancements with this expansion to make StarCraft II even more fun to play, and more exciting to watch. We're pleased to see gamers and eSports fans around the world enjoying Heart of the Swarm."
We said in our Starcraft 2: Heart of the Swarm review : "Heart of the Swarm is an accomplished, if not ground-breaking, addition to the StarCraft series.
"Should you buy HotS? Well, it's fun, simple, and will last you about 10-12 hours. If you've never played Starcraft... try the free demo for Wings of Liberty instead. If you've already tried that and liked it, HotS got the hots for you."