diff --git a/play.pokemonshowdown.com/src/battle-tooltips.ts b/play.pokemonshowdown.com/src/battle-tooltips.ts
index b13a4418b..2ea62eea7 100644
--- a/play.pokemonshowdown.com/src/battle-tooltips.ts
+++ b/play.pokemonshowdown.com/src/battle-tooltips.ts
@@ -1839,6 +1839,7 @@ export class BattleTooltips {
 			return 1;
 		}
 		const hardcoreMode = this.battle.hardcoreMode;
+		const inverse = this.battle.rules['Inverse Mod'];
 		const targetTypes = target.getTypeList();
 		const sourceAbility = source.effectiveAbility();
 		// Mold Breaker doesn't ignore _everything_, but it sure ignores everything that affects effectiveness
@@ -1905,7 +1906,15 @@ export class BattleTooltips {
 					factor = 1;
 					break;
 				}
-				factor = 0;
+				// Inverse replaces immunities with weaknesses. This has to
+				// be coded here, else it won't calculate secondary type's
+				// effectiveness. It sets a resistance to be consistent with
+				// the inversion done at the end.
+				if (inverse) {
+					factor *= 0.5;
+				} else {
+					factor = 0;
+				}
 			} else if (move.id === 'freezedry' && targetType === 'Water') {
 				factor *= 2;
 			} else {
@@ -1970,11 +1979,20 @@ export class BattleTooltips {
 			return factor * otherFactor === 0 ? 0 : 1;
 		}
 		if (hardcoreMode && dex.gen <= 9) {
-			if (factor > 2) return 2;
-			if (factor < 0.5) return 0.5;
+			if (factor > 2) factor = 2;
+			if (factor < 0.5) factor = 0.5;
+			if (inverse && dex.gen >= 7) return 1 / factor;
 			return factor;
 		}
-		if (hardcoreMode) return factor;
+		if (hardcoreMode) {
+			if (inverse && dex.gen >= 7) return 1 / factor;
+			return factor;
+		}
+
+		// Inverse Mod reverses effectiveness
+		if (inverse) {
+			return 1 / (factor * otherFactor);
+		}
 		return factor * otherFactor;
 	}
 	getMoveTypeText(move: Dex.Move, value: ModifiableValue, forMaxMove?: boolean | Dex.Move) {