StarCraft II: Heart of the Swarm is an expansion pack to the military science fiction real-time strategy game StarCraft II: Wings of Liberty, and the second part of the StarCraft II trilogy developed by Blizzard Entertainment, with the final part being Legacy of the Void. The game was released on March 12, 2013.

The expansion includes additional units and multiplayer changes from Wings of Liberty, as well as a continuing campaign focusing on the Zerg race and following Sarah Kerrigan in her effort to regain control of the swarm and exact her revenge on the Terran Dominion's emperor, Arcturus Mengsk.

During BlizzCon 2017, Blizzard announced that StarCraft II would be re-branded as a free-to-play game, hence opening the multiplayer mode to everybody and bringing some changes to previously paid features of the game. The Wings of Liberty campaign was made completely free while the campaigns for Heart of the Swarm and Legacy of the Void still required payment. However, those who had already bought Wings of Liberty before the free-to-play announcement were granted access to the Heart of the Swarm campaign free of charge. This new free-to-play model and changes to the availability of the campaigns was in line with Blizzard's vision to support the game differently going forward. Micro-transactions such as Skins, Co-op Commanders, Voice Packs, and the War Chests proved to be successful enough to sustain StarCraft II as a story-driven and eSport title.

Heart of the Swarm features a single-player campaign with 20 missions (plus seven evolution missions which allow the player to upgrade units), and continues the story from Wings of Liberty. The player plays from the perspective of Sarah Kerrigan, recently returned to her human form by Jim Raynor. Similar to Wings of Liberty, the briefing room allows interactive exploration, this time on the Leviathan, an enormous Zerg breed which functions as a bioship. Kerrigan and her allies are located in the nerve center. She has a personal chamber for altering her abilities, and there is an evolution pit where she can upgrade her units and perform evolution missions with the evolution master Abathur.

At BlizzCon 2011, it was revealed that Heart of the Swarm would feature seven new multiplayer units, while removing three units and changing the abilities of existing units and buildings. The exact modifications have since changed. In a blog post, game director Dustin Browder explained the current status of the units.

The Terrans were originally expected to feature two new units: the Shredder and Warhound. The Shredder was a mobile, burrowing turret based on the design of the Zerg Spine Crawler, attacking via clouds of toxic gas that were devastating to biological units. Internal testing revealed the Shredder was too flexible and powerful, and it was therefore replaced by the Widow Mine, a mobile burrowing unit that fires missiles, causing splash damage. The Warhound was a bipedal combat walker with an arm-mounted high-calibre cannon; it also possessed a missile for use against mechanical units. During the closed beta testing phase, pro gamers decried the Warhound for failing to function as intended; it was ultimately removed from multiplayer gameplay although it can be fought in the single-player campaign.

Several Terran units were modified in Heart of the Swarm. The Hellion, a four-wheeled buggy with a swivel-mounted flamethrower, gains the ability to transform into the Hellbat, a bipedal walker, whose flamethrower strikes in a fan shape instead of in a line. The Hellbat counts as both biological and mechanical, and can therefore be either repaired by SCVs or healed by Medivacs and Medics. The Medivac gains an "Ignite Afterburners" upgrade which acts as a cooldown-based speed boost. The Reaper has undergone extensive changes. It no longer does extra damage to light units, and its grenade, used only on buildings, has been removed entirely. It now has a passive health-regeneration ability that kicks in several seconds after the Reaper last took damage. Additionally, it no longer needs a Tech Lab add-on to be produced, allowing Heart of the Swarm players to train two of them at a time using the Reactor add-on.

The Protoss were originally expected to feature the Replicant, a unit which could transform into a clone of any other unit in play, including those controlled by the enemy. This unit was ultimately cut from the game because of its tendency to stifle unit diversity. Three new units made it into the game: the Oracle, Tempest, and Mothership Core.