Psychic is hands down one of the strongest Psychic-type moves, and it can be taught in Pokemon Brilliant Diamond and Shining Pearl via TM29.
TMs are spread all throughout the Sinnoh region in Brilliant Diamond and Shining Pearl. There are several of them found on the ground of buildings and routes as trainers continue on their journey.
The TM for Psychic isn't just one you'll come across, though. You will need to look for it, because you can completely miss it if you don't know where it is. In Brilliant Diamond and Shining Pearl, TM29 is found on Route 211.
Where to locate TM29 Psychic in Pokemon Brilliant Diamond and Shining Pearl
A lot of TMs can be purchased in Veilstone City at the department store. Unfortunately, TM29 Psychic is not one of them. You will have to find it the old fashioned way in Pokemon Brilliant Diamond and Shining Pearl.
Firstly, you will need to have learned the HM for Rock Climb and have it stored in your Poketch. To use it outside of battle, as part of your Poketch collection of HMs, you need to beat the Snowpoint City Gym Leader.
When all of that has been accomplished, go to Celestic Town and exit to the West. Go up the stairs near the sign indicating you are on Route 211. Rock Smash the rock behind the trainer.
Use Rock Climb to move up the cliff, and at the top will be the TM you've been searching for. Pick it up, and Brilliant Diamond and Shining Pearl will inform you that you've found TM29 Psychic.
The move comes with 90 Power and 100% Accuracy. A PP of 10 means it can be used 10 times in a row before it runs out, but that can be reset with certain items or by healing at a Pokemon Center.
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It's important to note that TMs are back to their old ways in Brilliant Diamond and Shining Pearl. Teaching one to a Pokemon will cause the TM to break and it will no longer be available in your inventory.