Outpacing the opponent by one point can flip a match. Jolly adds 10% Speed and lowers Sp. Attack, which physical attackers don't mind. For Red & Blue, Starmie, Arcanine, Scyther, and others all run Jolly in competitive play. There's no way to set natures in Red & Blue. You'll lock in Jolly when catching or breeding in Ruby/Sapphire or later.

Jolly raises Speed by 10% and lowers Sp. Attack by 10%. Fast physical attackers that need to outspeed threats. 63 Pokemon competitively run this nature.
#PokemonTierRoleAbility+Spe-SpASpe
1
Starmie
Starmie
WATERPSYCHIC
APhysical AttackerNatural Cure
115+16
100-12
115
2
Arcanine
Arcanine
FIRE
APhysical AttackerIntimidate
95+14
100-12
95
3
Scyther
Scyther
BUGFLYING
APhysical AttackerTechnician
105+15
55-8
105
4
Mew
Mew
PSYCHIC
ABulky SupportSynchronize
100+15
100-12
100
5
Gyarados
Gyarados
WATERFLYING
APhysical AttackerIntimidate
81+13
60-8
81
6
Sandslash
Sandslash
GROUND
APhysical AttackerSandrush
65+11
45-7
65
Starmie
Starmie
WATER PSYCHIC
A
Role
Physical Attacker
Ability
Natural Cure
+Spe
115 +16
Spe
115
Arcanine
A
Role
Physical Attacker
Ability
Intimidate
+Spe
95 +14
Spe
95
Scyther
Scyther
BUG FLYING
A
Role
Physical Attacker
Ability
Technician
+Spe
105 +15
Spe
105
Mew
Mew
PSYCHIC
A
Role
Bulky Support
Ability
Synchronize
+Spe
100 +15
Spe
100
Gyarados
Gyarados
WATER FLYING
A
Role
Physical Attacker
Ability
Intimidate
+Spe
81 +13
Spe
81
Sandslash
Sandslash
GROUND
A
Role
Physical Attacker
Ability
Sandrush
+Spe
65 +11
Spe
65
+10%Speed
-10%Sp. Attack
Flavor Preference
Likes Sweet, dislikes Dry
Competitive Role
Fast physical attackers that need to outspeed threats.
Jolly vs Adamant
Choose Jolly
+Speed / -Sp. Attack
  • Pokemon in contested speed tiers (base 80-110)
  • Sweepers that need to outspeed threats before attacking
  • Pokemon without priority moves or speed-boosting setups
Choose Adamant
+Attack / -Sp. Attack
  • Pokemon with priority moves that bypass speed
  • Dragon Dance / Shift Gear users that fix speed in-game
  • Wallbreakers that need maximum damage output
How to Get
SynchronizeLead with a Jolly Pokemon that has the Synchronize ability. Wild Pokemon will match its nature. (Gen 3+)
BreedingGive the parent with Jolly nature an Everstone. The offspring will inherit it. (100% since Gen 5)
Nature MintUse a Jolly Mint to change stat effects to Jolly. The original nature name stays but stats change. (Gen 8+)
Other +Speed Natures
Hasty (-Defense)Naive (-Sp. Defense)Timid (-Attack)
What does the Jolly nature do in Pokemon?
Jolly gives your Pokemon 10% more Speed at the cost of Sp. Attack. That tradeoff works best when the lowered stat isn't part of your moveset. Pokemon with Jolly nature prefer Sweet flavor and dislike Dry.
What are the best Jolly nature Pokemon?
Competitively, Starmie, Arcanine, Scyther, and Mew run Jolly most often. The +Speed boost complements their role, and the -Sp. Attack penalty barely affects their game plan.
Is Jolly a good nature for competitive Pokemon?
For the right Pokemon, Jolly is one of the best natures you can pick. It's one of the most-used natures in competitive formats. Just don't slap it on something that uses Sp. Attack-based moves.
How do I get a Jolly nature Pokemon?
Breeding with an Everstone is the most reliable approach since Gen 5. The parent holding it passes its nature to every offspring. Synchronize works for wild catches. And if you already have the Pokemon, a Jolly Mint from the Battle Tower changes the stat effects to Jolly.
Should I use Jolly or Adamant?
Short answer: Jolly for speed, Adamant for power. There's no universal best. Check your Pokemon's base stats and common threats in the format you're playing to decide which tradeoff is better.