Green Hill Zone, also known as Green Hill, is the first level of the platform game Sonic the Hedgehog, which released for the Sega Genesis in 1991. The level is grassy and lush, with environmental features such as palm trees, vertical loops and cliffs, and is the home of numerous forest animals. Like the game's other levels, Green Hill Zone comprises three acts; in the third, Sonic fights antagonist Doctor Eggman before moving to the second level, Marble Zone. It was constructed by level designer Hirokazu Yasuhara with its musical theme by Masato Nakamura.

Green Hill Zone is considered to be a classic video game level, akin to the first stage of Super Mario Bros., World 1-1. The level and its music have also received positive opinions from critics. It has appeared in other games in the series, such as Sonic Adventure 2, Sonic Generations, Sonic Mania, Sonic Forces, and Sonic Frontiers. Critics have noted a Green Hill Zone-like aesthetic in levels of other games.

Green Hill Zone is the first level of Sonic the Hedgehog. Located on South Island, it is a lush, grassy stage with unique features like droopy palm trees and crumbling cliffs. In addition, as with later levels in the game, Green Hill Zone has environmental features and obstacles like ramps, vertical loops, tunnels, spring-boards, spikes, and checkpoints. It is normally populated by woodland creatures, but antagonist Doctor Eggman imprisons them inside robots known as Badniks prior to the events of the game, so the player must destroy them to free the animals. Green Hill Zone consists of three acts, and the end of Act 3 features a boss battle with Robotnik. After beating Robotnik, Sonic moves on to the second level, the lava-themed Marble Zone.

Sonic the Hedgehog was created by the newly formed Sonic Team, a 15-member Sega subsidiary formed to create a character that could compete with Nintendo's Mario. The game's level design was handled by Hirokazu Yasuhara, and the musical theme was composed by Masato Nakamura, bassist from the J-pop band Dreams Come True. In designing Green Hill Zone, Yasuhara was inspired by the U.S. state of California, while the game's color scheme in general was influenced by the work of pop artist Eizin Suzuki.

The game's programmer, Yuji Naka, stated that it took him almost a year to create Green Hill Zone and that it was created and destroyed multiple times before arriving at the final version.

While the original game was a 2D side-scroller, Green Hill Zone was taken and remade in full 3D as a secret level in the 2001 game Sonic Adventure 2; the player unlocks it after collecting all 180 of the emblems found by completing the game's many objectives. The 2011 game Sonic Generations, a title that revisits past entries in the Sonic series, features both 2D ("Classic") and 3D ("Modern") versions of Green Hill Zone, as well as of numerous other Sonic levels. A reinterpretation of the level appears in the 2017 title Sonic Mania. Green Hill Zone reappears in Sonic Forces, having partially turned to desert due to resource depletion by Eggman's industries. Digital recreations of Green Hill Zone appear in the 2022 game Sonic Frontiers as part of the game's Cyber Space levels.

In addition, Green Hill Zone appears as a stage in the 2.5D fighting game Sonic Battle, in the tennis video game Sega Superstars Tennis, in the sports video game Mario & Sonic at the Sochi 2014 Olympic Winter Games, in the mobile title Sonic Dash, in the crossover adventure game Lego Dimensions, and in the crossover fighting games Dengeki Bunko: Fighting Climax and the Super Smash Bros. series. In the 2020 live-action film adaptation Sonic the Hedgehog, Green Hill Zone is depicted as Sonic's original home. The film series also features Sonic and friends living in a small town in Montana named Green Hills.

A Lego Ideas set based on the level was released on January 1, 2022. It received mixed reception from Alice Clarke of Kotaku, who called it "not the most thrilling build", remarking that its price was high, but that Sonic fans would love it.