In Pokémon Brilliant Diamond and Shining Pearl, Ditto is a must-have Pokémon for the breeders and shiny hunters amongst players. He can only be encountered post game, when you have become League Champion, but it isn't too hard to get your hands on the purple shape-shifter.
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How To Unlock Ditto
First things first, you will need to have beaten the Elite Four and Cynthia, thus crowning yourself the League Champion. Then, if you haven't already, you will need to complete your Sinnoh Pokédex by encountering every Pokémon in the region, including the illusive Drifloon.
After this, you can speak with both Professor Rowan and Professor Oak at the Pokémon Research building in Sandgem Town to have your Sinnoh Pokédex upgraded to the National Pokédex. Ditto will now be available to catch!
Where To Find Ditto
Ditto can be found in two places after having unlocked the National Pokédex in Brilliant Diamond and Shining Pearl.
Route 218
Make your way to Route 218, just west of Jubilife City, and Ditto can be found lurking in the tall grass here. You can use your Poké Radar to notify you of when a Pokémon is loitering, and then there is a 20% chance that the given Pokémon could be a Ditto!
Pokémon Hideaways
Alternatively, enter the Grand Underground and find either a Stargleam Cavern or Dazzling Cave. Ditto can be found hanging out in either of these two Pokémon Hideaways, so they're an easy catch!
That's where to find and how to catch a Ditto in Pokémon Brilliant Diamond and Shining Pearl! Ditto can breed all Pokémon except legendaries (excluding Manaphy), so, we recommend taking some time post-game to also explore Ramanas Park and attempt to get your hands on all the legendary Pokémon that Sinnoh has to offer.