What power-up allows Samus to roll into a ball?

The power-up that allows Samus to roll into a ball is the Morph Ball. It transforms her armored Power Suit into a compact sphere that can move through tunnels and other passages too small for her normal form.

The ability is central to Metroid's exploration design. In the original Metroid, it was called the Maru Mari, a Japanese name meaning roughly “round ball”; later games generally call it the Morph Ball or Morphing Ball. Once Samus obtains it, previously inaccessible parts of the map become reachable, helping establish the series' signature cycle of exploration, discovery, and backtracking.

The Morph Ball also supports several upgrades. Morph Ball Bombs let Samus destroy obstacles and perform bomb jumps, while the Spring Ball enables jumping in ball form. In the Metroid Prime games, the Boost Ball adds high-speed movement and lets Samus traverse half-pipes and activate special mechanisms. The Spider Ball, by contrast, is an expansion that lets her cling to certain surfaces; it is not the basic transformation itself.

The Morph Ball is a Power Suit ability, not a separate character or weapon. Its unusual design was partly practical: early developers found a rolling ball much easier to animate than a character crawling through narrow spaces.