Resident Evil Zero (or Resident Evil 0) is a 2002 survival horror video game developed and published by Capcom for the GameCube. It is a prequel to Resident Evil (1996), covering the ordeals experienced in the Arklay Mountains by special police force unit, the S.T.A.R.S. Bravo Team. The story takes place in July 1998 and follows officer Rebecca Chambers as well as convict Billy Coen as they explore an abandoned training facility for employees of the pharmaceutical company Umbrella. The gameplay is similar to other Resident Evil games, but adds the ability to switch between characters to solve puzzles and use unique abilities.

Development for Resident Evil Zero began for the Nintendo 64 in 1998. The partner system was created to take advantage of the short load times possible with the capabilities of the Nintendo 64 Game Pak. The cartridge format also provided limitations, as the storage capacity was significantly less than that of a CD-ROM. The team had to approach the design differently from previous series entries to conserve storage space. Resident Evil Zero was designed to be more difficult than previous Resident Evil games. Inspired by Sweet Home (1989), the team removed the item storage boxes present in earlier games and introduced a new item-dropping feature. Production was switched to the recently unveiled GameCube after development slowed due to memory storage issues. Only the concept and story remained from the original game, which had to be rebuilt.

Resident Evil Zero received generally positive reviews from critics, who praised the graphics and audio for building a haunting atmosphere. Opinions on the new partner and item systems were mixed. Some found the changes were an improvement and added new layers of strategy; others believed the changes were cumbersome or non-innovative. The game's tank controls were criticized as outdated.

The game was ported to the Wii in 2008, and a high-definition remaster was released in January 2016; the rereleases received mixed reviews due to lack of improvements. Resident Evil Zero was commercially successful, having sold over 4 million copies across all platforms.

Resident Evil Zero is a third-person survival horror video game. The gameplay remains largely the same as previous entries in the series. However, unlike controlling one sole character like the previous games, the player controls two protagonists throughout the game. The player may switch between police officer and medic Rebecca Chambers and convicted former Force Reconnaissance Officer Billy Coen. If they travel together, either one of them can be controlled while the other character is handled by the game's artificial intelligence (AI). The player may also control both simultaneously or split them up to explore areas separately. Each character has unique abilities. Rebecca has a mixing kit which allows her to combine herbs and other chemicals, but she is weak defensively. In contrast, Billy can move heavy objects, use a lighter, and has higher defense, but cannot mix herbs (a staple ability in Resident Evil games). The partner system is central to solving many of the game's puzzles.

The characters can run as well as perform quick 180-degree turns to evade danger. The player may examine objects such as doors, other characters, and items in order to find clues to proceed through the game. Some objects can be pushed or climbed upon to investigate higher levels. Items held by the characters can be examined in the inventory screen. Some items such as weapons can be equipped, and other items can be combined to create more effective items or replenish ammunition. When Rebecca and Billy are close by, they can exchange items between one-another. Previous series installments had the player store items in boxes placed in fixed locations. Resident Evil Zero has no item boxes, and instead allows players to drop items on the floor, freeing space in the inventory momentarily until they are retrieved at a later point. The locations of dropped items are displayed on the game map. The number of items that can be dropped in a room is limited.

On July 23, 1998, a train owned by the pharmaceutical company Umbrella, the Ecliptic Express, comes under attack from a swarm of leeches. As the passengers and crew are attacked, a mysterious young man watches from a hillside. Two hours later, the Bravo Team of the Special Tactics And Rescue Service (S.T.A.R.S.), a tactical unit of Raccoon City Police Department's (R.P.D.), is sent to investigate a series of cannibalistic murders in the Arklay Mountains outside of Raccoon City. On the way to the scene, its helicopter has an engine failure and crash-lands in a forest. Searching the area, officer Rebecca Chambers of Bravo Team comes across the Express, now motionless. She begins to investigate the scene, only to find the passengers and crew transformed into zombies. She is unaware their transformation is a result of exposure to Umbrella's T-virus contained within the leeches. As she explores the train, she teams up with Billy Coen—a former Marine Force Reconnaissance officer, who was to be executed for killing 23 people until the military police van transporting him crashed in the region.

The pair notices the mysterious young man, moments before the train suddenly begins moving again. Unbeknownst to the pair, two soldiers from Umbrella, on the orders of Albert Wesker and William Birkin, attempt to take control of the train and destroy it, but are killed by leeches before they can complete their mission. As the train speeds out of control, Rebecca and Billy apply the brakes and avert its course towards an abandoned training facility for future executives of Umbrella. They discover that the former director of the facility and the corporation's co-founder, Dr. James Marcus, was responsible for discovering the so-called Progenitor virus in the 1960s, and decided to examine its potential as a biological weapon. He combined it with leech DNA to develop the T-virus that causes rapid mutations in living organisms and thus transforms humans and animals into zombies and monsters.