Registeel locations — where to catch

In Sinnoh (Diamond, Pearl & Platinum)

• Iron Island — Walk
• Iron Ruins — Walk

In Hoenn (Ruby, Sapphire & Emerald)

• Ancient Tomb — Static Encounter

Other games (4)

• Sword / Shield — The Isle of Armor (Gen 8): Iron Ruins
• Ultra Sun / Ultra Moon (Gen 7): Ultra Space Wilds, Ultra Space
• Omega Ruby / Alpha Sapphire (Gen 6): Ancient Tomb
• Black 2 / White 2 (Gen 5): Underground Ruins

Registeel — full Pokédex entry