Sonic the Hedgehog is a Japanese video game series and media franchise created by Yuji Naka, Naoto Ohshima, and Hirokazu Yasuhara for Sega. The franchise follows Sonic, an anthropomorphic blue hedgehog who battles the evil Doctor Eggman, a mad scientist. The main Sonic the Hedgehog games are platformers mostly developed by Sonic Team; other games, developed by various studios, include spin-offs in the racing, fighting, party and sports genres. The franchise also incorporates printed media, animations, feature films, and merchandise.

Naka, Ohshima, and Yasuhara developed the first Sonic game, released in 1991 for the Sega Genesis, to provide Sega with a mascot to compete with Nintendo's Mario. Its success helped Sega become one of the leading video game companies during the fourth generation of video game consoles in the early 1990s. Sega Technical Institute developed the next three Sonic games, plus the spin-off Sonic Spinball (1993). A number of Sonic games were also developed for Sega's 8-bit consoles, the Master System and Game Gear. After a hiatus during the unsuccessful Saturn era, the first major 3D Sonic game, Sonic Adventure, was released in 1998 for the Dreamcast. Sega exited the console market and shifted to third-party development in 2001, continuing the series on Nintendo, Xbox, and PlayStation systems.

While Sonic games often have unique game mechanics and stories, they feature recurring elements such as the ring-based health system, level locales, and fast-paced gameplay. Games typically feature Sonic setting out to stop Eggman's schemes for world domination, and the player navigates levels that include springs, slopes, bottomless pits, and vertical loops. Later games added a large cast of characters; some, such as Miles "Tails" Prower, Knuckles the Echidna, and Shadow the Hedgehog, have starred in spin-offs. The franchise has crossed over with other video game franchises in games such as Mario & Sonic, Sega All-Stars, and Super Smash Bros.

Sonic the Hedgehog is Sega's flagship franchise and one of the bestselling video game franchises, selling over 140 million units by 2016 and grossing over $5 billion as of 2014. Series sales and free-to-play mobile game downloads totaled 1.5 billion as of 2022. The Genesis Sonic games have been described as representative of the culture of the 1990s and listed among the greatest of all time. Although later games, notably the 2006 series reboot, received poorer reviews, Sonic is influential in the video game industry and is frequently referenced in popular culture. The franchise is known for its fandom that produces unofficial media, such as fan art and fangames.