Which Metroid game was the first released on a handheld system?

The first Metroid game released on a handheld system was Metroid II: Return of Samus. Nintendo released it for the original Game Boy, beginning in North America in November 1991.

The game sends Samus to SR388, the home planet of the Metroid species. The Galactic Federation wants the creatures eliminated after earlier Space Pirate attacks, so Samus explores the planet's underground regions and hunts Metroids through several evolutionary stages. The game introduced series abilities such as the Space Jump, Spider Ball, and Spring Ball.

Metroid II was designed specifically for the Game Boy rather than being a shortened port of the NES original. Its monochrome presentation shaped several visual decisions, including the round shoulder plates later associated with Samus's Varia Suit. It also used battery-backed saving, a significant convenience for a large portable adventure.

Metroid Fusion and Metroid: Zero Mission were also handheld releases, but both arrived later on the Game Boy Advance. Metroid Prime Hunters was later still, appearing on the Nintendo DS. The official remake, Metroid: Samus Returns, was released for Nintendo 3DS in 2017.