No, you can only catch Registeel in Platinum if you have the event Regigigas. However, you can migrate it from the 3rd generation games using Pal Park. If you don't have a GBA game, then trade for it using GTS or trade with a friend that has it. It's the exact same thing for Regice and Regirock.
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You can not catch Registeel in these games. You have to catch him and the other regis on Pokemon Emerald, Ruby, or Sapphire and migrate them to Pal park your wrong go to route 228. there you will find it.
you must catch regice,regirock, and registeel
You can find Registeel in the Ancient Tomb on Pokemon Ruby, Sapphire and Emerald. It can also be found in the Underground Ruins of Pokemon White2. To get Registeel in Pokemon White2 you need to catch or defeat Regirock and use the Iron Key reward to unlock the Iron Chamber containing Registeel.
You cannot catch any of the Regis (Regirock, Regice, Registeel and Regigigas) in Pokémon HeartGold or SoulSilver.
Registeel is unobtainable from Pokemon Diamond and Pearl. You have to migrate it from Pokemon Ruby, Sapphire, or Emerald.