Nina Williams (Japanese: ニーナ・ウィリアムズ, Hepburn: Nīna Wiriamuzu) is a character from Namco's Tekken fighting game series. A cold-blooded professional assassin from Ireland, Nina made her first appearance in the original 1994 game. She is the only female playable character to appear in all main games of the Tekken series, and has starred in her own spin-off game, Death by Degrees.

Nina has an unstable and often violent relationship with her younger sister, Anna Williams, who is typically her archrival throughout the series. She is known for her fast and lethal fighting style, based loosely on fighting arts such as Koppo and Hapkido.

Nina has a blonde ponytail, blue eyes, and a slim hourglass figure, yet a powerful build. Her look contrasts that of her younger sister, Anna, who is a brunette. She dresses primarily in purple fitted costumes and is commonly seen wearing a military-style outfit to suit her occupation. According to Namco's original Tekken design team, "It was Nina or Kazuya who was the soul, the cool part, of the [first] game." Producer Yozo Sakagami said about their choice of Roberto Ferrari as her character designer, "Nina is a white character, and as you can see with games like Final Fantasy, when Japanese artists try to design white women or whatever, [the characters] kind of turn Japanese in their facial features and just the way they look, generally. And we didn't want that with Nina."

When developing Tekken 4, Harada conceived her like a biker with a designed elaborated based on Lupin III character Fujiko Mine, and designed by lead illustrator, Takuji Kawano. The suit was originally going to completely black but due to issues with the PlayStation 2's visibility it was hard to see. This led to the artist doing a pink suit instead fit with a knife for combat in her right leg. In Tekken 6, one of Nina's extra outfits is one of her Death by Degrees costumes. Her default costume in Tekken 7 is a wedding gown: off-white, burned and tattered, long strapless wedding gown and veil, white gloves and stockings, and white stiletto heels. In Tekken 8, Nina is shown wearing an unzipped black leather jacket, with a purple dress underneath, accompanied with black sunglasses.

Nina is known for her lethal grappling techniques and counterattacks. She also has a versatile repertoire of striking attacks with all limbs. Some of her moves are shared with her sister Anna, especially in the first two Tekken games. Nina was also among the first characters to utilize chain throws, having them since Tekken 2 where she was recommended for aggressive players. In Tekken Tag Tournament, she was noted as "feared by many players" due to her many combos that are difficult to counter, but her command list has a high execution barrier; she is a challenge, requiring a lot of practice to master. In Tekken 6, she has strong jabbing abilities, is agile and has above average strength.

Nina, a silent assassin trained by her parents, is sent to eliminate Heihachi Mishima in the original Tekken (1994). Later in Tekken 2 (1995), she is tasked with assassinating Kazuya Mishima, but is captured along with her sister Anna and used in a 15-year cryogenic experiment. Awakening in Tekken 3 (1997) with amnesia, Nina is controlled by the creature Ogre. In Tekken 4 (2001), Nina is contracted to kill British boxer Steve Fox, but abandons the mission upon discovering he is her son. Tekken 5 (2004) features her ongoing rivalry with Anna, where they engage in a gun battle, before agreeing to participate in the tournament to settle their rivalry, while in Tekken 6 (2007), she serves as Jin's bodyguard and antagonist. In Tekken 7 (2015), she works for Heihachi before resigning and returning to freelance assassinations, where she assassinates Anna's to-be husband, who had darker ties with the syndicate, and reveals everything she knows to Steve about their relations to the Mishima Zaibatsu's abandoned past project. In Tekken 8 (2024), she takes over Anna's former position as Kazuya's bodyguard, following her sister's expulsion from G Corporation.

Nina is the protagonist of Death by Degrees, an action-adventure spin-off of the Tekken games. She has also been featured in other series spin-offs such as Tekken Tag Tournament, Tekken Card Challenge, Tekken Tag Tournament 2, Tekken Revolution, and Tekken Mobile, and the crossover games Street Fighter X Tekken and The King of Fighters All Star. Additionally, she appears in the 1998 animated film Tekken: The Motion Picture, the 2009 Tekken film adaptation, the 2011 animated film Tekken: Blood Vengeance, and the 2022 anime Tekken: Bloodline. In Tekken Mobile, Nina also has summer-themed variant named Summer Nina.

Summer Daniels portrayed Nina in the Tekken Tag Tournament 2 live-action trailer "Girl Power" in 2012. In 1998, Epoch Co. released a 1/10 scale Nina action figure based on her appearance in Tekken 3, including two plastic swords and one plastic gun. An action figure based on her appearance from Tekken 5 was released in 2006; two figures based on her appearance in Death by Degrees were also released that same year. Kotabukiya released a bishōjo figure of Nina from Tag Tournament 2 designed by Shunya Yamashita for the 20th anniversary of Tekken in 2014. In 2017, Nina was one of the five Tekken characters to receive a Funko Pop.