Ken Masters (Japanese: ケン・マスターズ, Hepburn: Ken Masutāzu) is a character in Capcom's Street Fighter fighting game series. The character was first introduced in the 1987 title Street Fighter as an alternate fighter to Ryu. While Ryu is devoted to testing his power, Ken instead focuses on developing a family with his wife Elisa. Ken becomes a mainstay in the following Street Fighter games. An alternate brainwashed version of him, Violent Ken (暴力的なケン), has also been featured in a few of his appearances. Ken's presence is in several sequels to the Street Fighter series in addition to its related media. He has also made crossover appearances in Namco × Capcom, Project X Zone, and Super Smash Bros.

Capcom designer Takashi Nishiyama created Ken when developing the first Street Fighter game. However, due to budget problems, Ken could only use Ryu's moves, which later resulted in their characterization as friendly rivals and fellow students of Gouken. Shoei Okano eventually redesigned the character in later games. Seeking to make the character more unique, Capcom redesigned Ken's red gi in Street Fighter V and Street Fighter 6, though the latter resulted in changing his entire character depicting his life as less stable.

Despite being recognized as a clone of Ryu, Ken develops a more social life rather than devoting his life to his training like Ryu. Their similarities inspired the term Shotokans in other fighting games besides Street Fighter. The evolution of his gameplay and design has been popular among gamers for becoming more unique. However, some critics were critical of his role in Street Fighter 6, which was significantly changed due to the sudden increase in depression resulting from losing his family.

Capcom designer Takashi Nishiyama created Ken's character as he became Street Fighter director when coming up ideas with a project. Inspired by Spartan X, Nishiyama took notes about the needs of Street Fighter. Planner Hiroshi Matsumoto was influenced by martial arts which led to the usage of multiple styles. Nishiyama thinks the most regrettable experience of the game was the fact that, they had all these ideas for coming up with different playable characters that they wanted to put into the game, but due to budget concerns, scheduling concerns, among others, eventually that got reduced to only Ken and Ryu. Nishiyama would have loved to have more playable characters, but Capcom were only able to put in just the two of them.

In Street Fighter II, Ken and Ryu were originally going to be Y.S.'s characters; however, he ended up taking over in the early stages. Okano remembers making the dot patterns for the Shoryuken and the Hadoken special moves first. After that, he moved away from game design and created a design room for the illustrations and manuals. Akiman recalls having drafts for most characters except Ken and Ryu. Y.S. eventually finished the characters. From there, the first move he made was the Hadoken. Ryu and Ken "are the so-called standard characters in the" game, and since they were coming from the first Street Fighter game, they could have the other characters be "weirdos". He gave them a stoic feel. Numerous spinoff products were made as well during the game's popularity: when Capcom licensed Hasbro to produce a line of action figures, Ken was given the surname "Masters" to avoid trademark conflict with Mattel's Ken dolls. The full name Ken Masters was used in the animated Street Fighter II movie and in the Street Fighter II V series before being canonized in the video games with Street Fighter Alpha 2.

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 we 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 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. Yasuda disliked the handling of Ryu and Ken as old characters, resulting in the creation of Sakura Kasugano as a response to a younger, cooler fighter that contrast the others especially in outfits. In the prequel Street Fighter Alpha Capcom did not want to end up with just Japanese and Asian characters. As much as we 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. With Street Fighter IV, the producer said that while Ryu comes across as an ordinary man, Ken contrast him with a more well-defined face. Furthermore, they said he has "a cool, fun-loving guy." which made him easier to design.

In both SNK vs. Capcom: SVC Chaos and Street Fighter II, game designers created an alternate persona called "Violent Ken" with stronger moves than the regular Ken Masters. Despite similarities with Ryu's evil persona caused by not being able to control his dark powers, Violent Ken is the result of the villain M. Bison manipulating Ken with his Psycho Power. This exploits Ken's hidden side including fears or hatred, most notably his inferiority complex over Ryu and starts wielding purple flames alongside all of his attacks. Eventually, Ken recovers his original persona when seeing his best friend's red bandana which is a present the character gives him in Street Fighter Alpha 2. Though Ken did not undergo several changes in Street Fighter IV, he and Ryu Rufus were meant to be have an acrobatic bald slender black male wearing a gi named "King Cobra", meant to be a more, the character became Rufus. Reuben Langdon voiced Ken in Street Fighter IV, Super Smash Bros. Ultimate and Street Fighter V. David Matranga took over in Street Fighter 6, feeling responsibility for it as a result of the series' popularity. He also regarded Ken as iconic based on how he gets the chance to yell his attack's names which required lessons of screaming and grunting.

The character's inclusion in the SNK fighting game Fatal Fury: City of the Wolves over Ryu was done because producer Yasuyuki Oda said in the Capcom vs. SNK crossover games, Ken's character was often promoted alongside Terry Bogard, who originated in Fatal Fury in contrast to Ryu who instead is marketed alongside Kyo Kusanagi who originated in The King of Fighters '94. This reasoning was also done with fellow character Chun-li who tends to be promoted alongside Mai Shiranui.