Ryu (/riˈuː/ ^ⓘ; Japanese: リュウ, Hepburn: Ryū) is a character and the protagonist of Capcom's Street Fighter series. Introduced in Street Fighter (1987), Ryu appears as the game's lead character alongside his best friend and friendly rival Ken Masters. Other games in the series show Ryu's training and dedication to being the strongest fighter he can be, befriending new fighters. Unable to control his dark nature, Ryu develops two alter egos throughout the series: Evil Ryu (殺意の波動に目覚めたリュウ, Satsui no Hadō ni Mezameta Ryū), a version of the character with his evil intent awakened, and Kage (影ナル者, Kagenaru Mono), a separate entity who is the physical embodiment of that evil intent. Mastering the dark nature is Ryu's main objective in order to become stronger. He has appeared as a playable character in several crossover game franchises, including Marvel vs. Capcom, SNK vs. Capcom, Project X Zone, and Super Smash Bros. Additionally, he is featured in manga and anime adaptations, as well as the 1994 live-action film.

Ryu was created by game designer Takashi Nishiyama, who was inspired by the South Korean-born martial artist Mas Oyama while designing the character. For his second appearance, Ryu's design changed from a young fighter to a skilled karate practitioner. However, because of issues in the making of Street Fighter II, he possessed a major weakness within the cast. For the next titles, Ryu's fighting style was modified, so he had different skills, with Evil Ryu and Kage possessing more diverse moves. Multiple Japanese and English actors have voiced him. While his appearance remained mostly intact, Street Fighter 6 gave Ryu a major redesign in reference to his master Gouken.

A pop culture icon, Ryu has become one of the most iconic fighting game characters ever, inspiring several other fighters through his alter egos while his friendship and rivalry with Ken were praised for how different they developed despite being nearly the same in their introductions. The character also received mixed responses for recycling an archetype of the corrupted protagonist in the form of Evil Ryu and Kage. In Street Fighter 6, Ryu's appearance attracted critics for his sex appeal.

Before the creation of Ryu as the main protagonist of the first Street Fighter game, Capcom designer Takashi Nishiyama had initially envisioned a different character for the role. The original concept was for a Caucasian man donning a leather jacket, who even makes an appearance during the opening sequence of the game. However, this idea was eventually set aside in favor of creating a character who practiced karate and wore a traditional dougi, leading to the creation of Ryu. Still wanting to utilize the concept, after subsequently transitioning to SNK, Nishiyama later implemented the character as Terry Bogard, who debuted as the protagonist of 1991's Fatal Fury: King of Fighters.

Nishiyama's inspiration for Ryu was Mas Oyama, who was depicted as the protagonist in the kung fu manga and anime series Karate Master by Ikki Kajiwara. Nishiyama grew up watching the animated series, which was influenced by Oyama's life. Nishiyama was impressed by Oyama's martial arts skills and philosophies, and this inspired him to create the first Street Fighter game. Nishiyama wanted the game to have a story similar to a film, anime, or novel. Whereas Ryu was based on Oyama, his rival Sagat was also influenced by the protagonist's rivalry with a bald Muay Thai martial artist from the series. Ryu's name was based on Nishiyama's name because the on'yomi (Sino-Japanese pronunciation) of the character "Takashi" is "Ryū" (Mandarin: Lóng, 隆).

In an interview with Game On!, Capcom Research and Development head Noritaka Funamizu said that of the series' characters, Ryu was one of the most popular with American audiences, alongside Zangief and Guile. Ryu's Shoryuken was noted to be highly difficult to execute, something which the team wanted to fix. Although Ryu and Ken had the same moves, Yoshiki Okamoto said Ryu was the weaker of the two. This was because Capcom wanted to add a weak point to their Shoryukens. They later decided not to do so, but a mistake resulted in Ryu becoming the weaker character. Ryu and Ken were mainly handled by Shoei, who had clear memories of designing the Hadoken and Shoryuken. Shoei Okano recalled they were originally going to be Y.S.-san's characters in the early stages of development. He noted Ryu and Ken are the "so-called standard characters in the SFII world, and since they were coming from SFI, we could have the other characters be weirdos." As a result, the duo was redesigned to have a stoic feel.

The first decision when making the prequel Street Fighter Alpha was keeping Ken, Ryu and Chun-Li. Capcom did not want to end up with just Japanese and Asian characters. As much as they could, they wanted to think in terms of creating a good balance, with all sorts of different skin colors. The secret of Street Fighter Alpha is the Ryu and Ken vs. Bison team-up mode, where if word did not get out, we figured we would just stay silent. However, the staff spoiled it. While they found such an idea fun, it eventually became a troublesome mode. This eventually inspired more team-up fights involving characters who wear opposing colors like Ryu and Ken. While Street Fighter keeps giving Ryu's quest for self-improvement, Alpha 2 introduced the concept of a darker alter-ego influenced by Akuma, Evil Ryu.

Ryu wears a white gi, as a symbol of his Japanese heritage, with the Japanese symbols for the Fūrinkazan (風林火山 (Hepburn: "Wind, Forest, Fire, Mountain")) on his belt as well as a stylised version on his gloves. While many Street Fighter characters have been redesigned, Ryu's design has stayed largely the same. In Street Fighter II, the character was included because of presence in the first game, symbolizing the concept of a Japanese martial artist. As the series progressed, his design was made more muscular to coincide with the concept, while his white gi, considered his most defining characteristic by the development team, let viewers know he was "a karate master at first sight". For the Street Fighter Alpha games, Ryu was redesigned as a younger character. Artist Naoto "Bengus" Kuroshima noted that the expectations that come with drawing him or Chun-Li were greater compared to newcomers like Sakura Kasugano. When picking the cast of Street Fighter V, producer Yoshinori Ono said he wanted Ryu and the cast to differ from their previous characters although Ryu retained his original look in contrast to Ken.