diff --git a/src/data/mystery-encounters/encounters/shady-vitamin-dealer-encounter.ts b/src/data/mystery-encounters/encounters/shady-vitamin-dealer-encounter.ts
index 798e85868f6..3f1ace47b0f 100644
--- a/src/data/mystery-encounters/encounters/shady-vitamin-dealer-encounter.ts
+++ b/src/data/mystery-encounters/encounters/shady-vitamin-dealer-encounter.ts
@@ -138,7 +138,7 @@ export const ShadyVitaminDealerEncounter: MysteryEncounter =
             newNature = randSeedInt(25) as Nature;
           }
 
-          chosenPokemon.customPokemonData.nature = newNature;
+          chosenPokemon.setCustomNature(newNature);
           encounter.setDialogueToken("newNature", getNatureName(newNature));
           queueEncounterMessage(scene, `${namespace}:cheap_side_effects`);
           setEncounterExp(scene, [ chosenPokemon.id ], 100);
diff --git a/src/data/mystery-encounters/encounters/training-session-encounter.ts b/src/data/mystery-encounters/encounters/training-session-encounter.ts
index ce3a89ffc7c..2e134c0b442 100644
--- a/src/data/mystery-encounters/encounters/training-session-encounter.ts
+++ b/src/data/mystery-encounters/encounters/training-session-encounter.ts
@@ -225,9 +225,8 @@ export const TrainingSessionEncounter: MysteryEncounter =
           const onBeforeRewardsPhase = () => {
             queueEncounterMessage(scene, `${namespace}:option.2.finished`);
             // Add the pokemon back to party with Nature change
-            playerPokemon.setNature(encounter.misc.chosenNature);
-            playerPokemon.customPokemonData.nature = encounter.misc.chosenNature;
-            scene.gameData.setPokemonCaught(playerPokemon, false);
+            playerPokemon.setCustomNature(encounter.misc.chosenNature);
+            scene.gameData.unlockSpeciesNature(playerPokemon.species, encounter.misc.chosenNature);
 
             // Add pokemon and modifiers back
             scene.getPlayerParty().push(playerPokemon);
diff --git a/src/field/pokemon.ts b/src/field/pokemon.ts
index 78f82e929de..3a8ab2f6bf3 100644
--- a/src/field/pokemon.ts
+++ b/src/field/pokemon.ts
@@ -1067,6 +1067,11 @@ export default abstract class Pokemon extends Phaser.GameObjects.Container {
     this.calculateStats();
   }
 
+  setCustomNature(nature: Nature): void {
+    this.customPokemonData.nature = nature;
+    this.calculateStats();
+  }
+
   generateNature(naturePool?: Nature[]): void {
     if (naturePool === undefined) {
       naturePool = Utils.getEnumValues(Nature);
diff --git a/src/modifier/modifier.ts b/src/modifier/modifier.ts
index 0891262649c..281cd94d05d 100644
--- a/src/modifier/modifier.ts
+++ b/src/modifier/modifier.ts
@@ -1,5 +1,5 @@
 import type BattleScene from "#app/battle-scene";
-import { FusionSpeciesFormEvolution, pokemonEvolutions, pokemonPrevolutions } from "#app/data/balance/pokemon-evolutions";
+import { FusionSpeciesFormEvolution, pokemonEvolutions } from "#app/data/balance/pokemon-evolutions";
 import { getBerryEffectFunc, getBerryPredicate } from "#app/data/berry";
 import { getLevelTotalExp } from "#app/data/exp";
 import { allMoves } from "#app/data/move";
@@ -2189,14 +2189,8 @@ export class PokemonNatureChangeModifier extends ConsumablePokemonModifier {
    * @returns
    */
   override apply(playerPokemon: PlayerPokemon): boolean {
-    playerPokemon.customPokemonData.nature = this.nature;
-    let speciesId = playerPokemon.species.speciesId;
-    playerPokemon.scene.gameData.dexData[speciesId].natureAttr |= 1 << (this.nature + 1);
-
-    while (pokemonPrevolutions.hasOwnProperty(speciesId)) {
-      speciesId = pokemonPrevolutions[speciesId];
-      playerPokemon.scene.gameData.dexData[speciesId].natureAttr |= 1 << (this.nature + 1);
-    }
+    playerPokemon.setCustomNature(this.nature);
+    playerPokemon.scene.gameData.unlockSpeciesNature(playerPokemon.species, this.nature);
 
     return true;
   }
diff --git a/src/system/game-data.ts b/src/system/game-data.ts
index 4b7eeedfe77..ede7bd0c986 100644
--- a/src/system/game-data.ts
+++ b/src/system/game-data.ts
@@ -1791,6 +1791,35 @@ export class GameData {
     });
   }
 
+  /**
+   * @param {PokemonSpecies} species
+   * @returns {boolean} whether root species has been unlocked for given {@linkcode PokemonSpecies}
+   */
+  isRootSpeciesUnlocked(species: PokemonSpecies): boolean {
+    return !!this.dexData[species.getRootSpeciesId()]?.caughtAttr;
+  }
+
+  /**
+   * Unlocks nature for a given {@linkcode PokemonSpecies} and its prevolutions.
+   * Will fail silently if root species has not been unlocked
+   * @param {PokemonSpecies} species
+   * @param {Nature} nature
+   */
+  unlockSpeciesNature(species: PokemonSpecies, nature: Nature): void {
+    if (!this.isRootSpeciesUnlocked(species)) {
+      return;
+    }
+
+    //recursively unlock nature for species and prevolutions
+    const _unlockSpeciesNature = (speciesId: Species) => {
+      this.dexData[speciesId].natureAttr |= 1 << (nature + 1);
+      if (pokemonPrevolutions.hasOwnProperty(speciesId)) {
+        _unlockSpeciesNature(pokemonPrevolutions[speciesId]);
+      }
+    };
+    _unlockSpeciesNature(species.speciesId);
+  }
+
   updateSpeciesDexIvs(speciesId: Species, ivs: integer[]): void {
     let dexEntry: DexEntry;
     do {