To upgrade your Pokédex to the National Mode, you will first need to completely fill the Sinnoh Pokédex by seeing every possible Pokémon in Sinnoh (owning them all is not required).
After you’ve accomplished that, speak to Professor Rowan in Sandgem Town and he will upgrade your Pokédex to the National ‘Dex.
Step 1: Completing the Regional Pokédex
The first step to obtaining the National Pokédex is to fill out every page of the Sinnoh Dex.
You don’t need to catch all 210 Pokémon that are native to the Sinnoh region in order to accomplish this, but you’ll need to observe them at least once.
This can be achieved by:
- Capturing them
- Battling them in the wild
- Facing them in a trainer battle
- Or even reading about them in books
This isn’t as arduous a task as it sounds at first.
By following a general rule of battling every trainer you encounter throughout the game, the vast majority of Pokédex pages will be filled out by the time you complete the main story.
With that said, there are a few species that players often miss during their initial playthrough, since no trainers use them in battle.
The following is a list of some of the more elusive Pokémon you’ll need to find before you can upgrade to the National Pokédex.
Unown can only be found in the Solaceon Ruins, which lie to the east of Solaceon Town.
You will visit the Solaceon Ruins at least once in the game, but Unown won’t start showing up until after you’ve picked up HM05 Defog in the final cave chamber.
At that point, many players simply use an Escape Rope to leave the cave.
But if you roam around the ruins instead of using an Escape Rope, you’ll start running into Unown in random encounters.
#146, #147, and #148: Uxie, Mesprit, and Azelf
You will catch glimpses of the three lake guardians throughout the game. But you won’t be able to register any of them to the Pokédex until after the showdown with Cyrus in the Distortion World.
Once you’ve defeated Cyrus, each lake guardian will return to the caverns in the middle of their respective lakes.
The game won’t direct you to visit these three Pokémon, so some players forget to do so.
Here’s where to find these three:
- Uxie is inside Acuity Cavern
- Mesprit is in Verity Cavern
- Azelf is in Valor Cavern
You’ll likely want to capture the lake trio instead of knocking them out.
But as long as you interact with each Pokémon once, they will be entered into the Pokédex.
Manaphy is a Pokémon many players miss out on, simply because it requires a game in the Pokémon Ranger series.
However, a book in the Pokémon Mansion on Route 212 has the information required to add Manaphy to the Pokédex.
You’ll find the book in Mr. Backlot’s office in the eastern wing.
A lot of players miss out on getting Rotom simply because they don’t visit its only location at the right time of day.
There is a single Rotom to be found in the entire game. It haunts the Old Chateau in Eterna Forest, and you’ll only be able to make it appear by visiting at night.
At any time between 8:00 PM and 3:59 AM, interact with the television on the Old Chateau’s second floor to be prompted to hit the TV and summon Rotom.
Be very careful not to knock Rotom out—you will not get another chance to catch it.
Step 2: Acquiring the National Pokédex
Once you’ve filled your Pokédex with all 210 Pokémon from the Sinnoh region, it’s time to head back to Professor Rowan’s lab in Sandgem Town.
Speak to Professor Rowan and Professor Oak will show up and greet you both.
Rowan will brag about your accomplishment in filling the Pokédex, which will intrigue Professor Oak.
Oak will then ask for your help in documenting all the world’s Pokémon. He will then upgrade your Pokédex to include room for the entries of every Pokémon from Generations I, II, and III.
After Professor Oak leaves, Professor Rowan will award you the Poké Radar.
This item will help you find many of the new Pokémon you’ll be able to find now that you have the National Pokédex upgrade.
National Pokédex Details
With the upgrade to the National Dex, you will now be able to document and obtain all of the Pokémon from previous generations, bringing the total number up to 493.
Note that not every one of these Pokémon can be found in Pokémon Platinum. Many can only be received by other means, such as trading with other games.
The National Pokédex will also open up the rest of the Battle Zone and Pal Park, as well as Swarms.