The PlayStation 2 version of Half-Life is a port of the Windows game, but with a significant overhaul for the weapons and character models, as well as the overall map geometry and some extended levels.

It also includes an expansion pack that is referred to as Half-Life: Decay, which was never officially available as an expansion for the Windows game until it was eventually recreated as a modification. The third expansion for the original game, it features two-player cooperative gameplay (in split-screen) with the two female scientists Gina Cross and Colette Green as the protagonists. It is meant to be played with two players at the same time as it is more puzzle-oriented than the original game and the first two expansions. Single player is also possible, but then switching between the two characters is necessary to advance. When not active, the other character is AI controlled for basic actions.

It is set at the same time of the events in the original Half-Life, but from the perspective of other characters. All enemies and weapons are based on the original game and many of the missions have a more friendly approach about working together or rescuing NPCs. Unique to the series as a whole is the lack of a single narrative; instead the add-on consists of fixed, separate missions. At the end of each mission, statistics are shown and the player is graded. When the nine levels are completed with an A grade, a bonus mission is unlocked where players control and work together as two Vortigaunts. As an additional bonus, the entire single player can then also be completed as a Vortigaunt.

The game supports an USB mouse and keyboard for more precise control.