Sub-Zero is the name of two fictional characters in the Mortal Kombat fighting game franchise by Midway Games and NetherRealm Studios. Warriors from the fictional Lin Kuei clan, the two possess the ability to control ice in many forms. Characters by the name are the only fighters to appear in every main installment of the series, along with the action-adventure spin-off Mortal Kombat Mythologies: Sub-Zero (1997).

In the games, the mantle of Sub-Zero is used by brothers Bi-Han (Chinese: 避寒; pinyin: Bì Hán) and Kuai Liang (Chinese: 奎凉; pinyin: Kúi Liáng). Bi-Han debuted as Sub-Zero in the Mortal Kombat before appearing in subsequent installments as Noob Saibot. Kuai Liang took over the mantle in Mortal Kombat II (1993) and served as the franchise's main Sub-Zero up to Mortal Kombat: Onslaught (2023). In Mortal Kombat 1 (2023), Bi-Han returns as Sub-Zero, while Kuai Liang becomes Scorpion.

One of the Mortal Kombat franchise's signature characters, Sub-Zero has appeared in various related media outside of the games. He has received critical acclaim for his appearance, abilities, and Fatality finishing moves.

Sub-Zero was first conceived by Mortal Kombat co-creator John Tobias as a mysterious character named simply "Ninja" and inspired by the book China's Ninja Connection by Li Hsing, which "posits historical evidence for the existence of the Lin Kuei and their influence on the Japanese ninja. I was aware of there being some controversy about the author’s claims. So when we split the character in two for palette swaps, I thought it would be fitting that one was of Chinese origin and the other Japanese to kind of embody the argument." According to Tobias, the original "Hunter&Hunted concept was going [to be] about a ninja escaping from his clan and hunted by another member. We used that later for SZ in MK3." Sub-Zero's early name was Tundra.

In his early appearances, Sub-Zero was primarily portrayed and voiced by non-Asian actors. Midway Games later explained Sub-Zero's rather occidental appearance for a Chinese assassin by giving him a white mother. According to this new backstory, his father had a wife, two sons, and a daughter while he lived in America to hide his personal role as an assassin for the Lin Kuei. Midway Games programmer Josh Tsui portrayed the unmasked Sub-Zero (Kuai Liang) in the character's Mortal Kombat II ending.

Sub-Zero's early costumes have been very simple due to technical limitations. Mortal Kombat co-creator Ed Boon noted that Kuai Liang's counterpart from DC Comics is Batman as both are "dark, mysterious, brooding characters". Since Mortal Kombat 3, Sub-Zero has had a scar running down from his forehead and across his right eye as a mark of death. The scar was originally red, and later changed to blue in Mortal Kombat: Deadly Alliance as a result of Sub-Zero's enhanced powers. Tobias said that Sub-Zero was unmasked in Mortal Kombat 3 in order to stir up fan speculation about the character's backstory. The new Sub-Zero made his official debut on the cover of GamePro April 1995 issue, with Turk's red outfit tinted blue (as it was for the game), but the photo used was a reversed negative, as his scar was over his left eye.

Mortal Kombat and Deadly Alliance states that Sub-Zero is 6 feet 2 inches tall; in recent games such as Mortal Kombat X and Mortal Kombat 11, his sprite appears to be similar to most characters. In Mortal Kombat: Deception, his scar has faded to the point where it can no longer be seen. He now wore an armored uniform heavily inspired by Dynastic era Chinese battle armor, though it was often criticized by fans as being too reminiscent of Teenage Mutant Ninja Turtles villain, the Shredder. However, Sub-Zero's alternate uniform was very reminiscent of Chinese culture, even featuring Sub-Zero without a cowl and a Qing dynasty-era hairstyle. In Mortal Kombat: Armageddon, Sub-Zero reverts to his masked costume from Deadly Alliance, retaining the scar, which is not part of his alternate costume (which is the primary one from Deception). Since Deadly Alliance, Sub-Zero is the only character who has shown considerable signs of aging. Concept art from Deadly Alliance depicted him with a graying, receding hairline, and a more pale and gaunt face, while his scar was now blue and his forearms frozen over. Although Sub-Zero originally had blue eyes, they turned white after he obtained the Dragon Medallion in the game. Sub-Zero's appearance in Mortal Kombat: Shaolin Monks was one of the most revised ones from the title. Character lead Mark Lappin did almost ten passes on his design; producer Shaun Himmerick noted that "we went through literally 5-6 heads and styles of head costume on him" and commented that Sub-Zero's design in Mortal Kombat was difficult to make although most people called it "simple". In the end, the staff was satisfied with his final look in the game as it reminisces them to his classic outfit. His appearance in Injustice 2 was redesigned by Jim Lee.

When he first appeared in the first Mortal Kombat, Sub-Zero featured only two special moves: his ice blast and sliding kick. These moves have become Sub-Zero's trademark since then, being featured in every game that Sub-Zero has appeared in (Mortal Kombat II and subsequent games feature a younger Sub-Zero whose identity is Kuai Liang). Mortal Kombat II added his ground freeze move, and two new Fatalities including the now-famous one where he would freeze and shatter the victim. According to Boon, Sub-Zero's Freeze was originally omitted from the game in place of the Ice Shower, but was brought back in the next revision following fan feedback at a local arcade. Sub-Zero's Predator-inspired Fatality, the "Spine Rip," is considered by Boon to be his favorite Fatality from the first game as well as the most controversial.