“Luigi, I found this cap in front of the castle a little while ago. You must have dropped it. If you want to become a full-fledged hero like your brother, you can't be so careless!”
—Toad, Super Mario 64 DS
with its signature green "L" emblem.
Luigi's Cap is one of Luigi's important pieces of clothing. Luigi has had his hat in hand for his every appearance with no exceptions. Luigi has worn the hat since he was a baby.
Luigi's hat was meant to contradict Mario's red cap since green is the opposite color of red. The hat was made green with a green "L" emblem.
During the events of Super Mario Bros. 2, the tip of the cap had been changed to Black instead of his usual green, this happened for Mario as well. However, after this game, Luigi's cap went back to having a green tip.
In the Mario Kart games Mario Kart 64 and Mario Kart Wii, in the Luigi's Circuit (Luigi's Raceway in Mario Kart 64) courses, giant Luigi's caps can be seen on top of the grandstands.
In the Mario & Luigi series, the "L" on Luigi's cap is black. Mario's cap was changed in the same way, but outside of the Mario & Luigi series, the "L" on his cap is never black.
In the game Super Paper Mario, when Luigi is reverted to Mr. L, his hat takes a slight change with the white on his hat being turned to black and the "L" in the middle being turned backward.
In Super Mario 64 DS, Luigi's hat was used to shape-shift into Luigi if the player was either Mario, Wario, or Yoshi. In the same game, Luigi is affected the same way as Mario and Wario if their hats are lost. Luigi's defense and attack sharply decline, while his hat is absent from his head. If the player gets the hat back, Luigi's attack and defense revert back to normal. If Luigi is locked, it only appears in Big Boo's Haunt for any star except #1.
 Cap-less Luigi
Sometimes, Luigi will be seen without his cap on.
- In Luigi's Mansion when Luigi gets hit by a bomb or fire, his hat will catch flame and he will take off his hat and shake it. Thus, he is seen hat-less.
- Another time where he has no hat on is in New Super Mario Bros. when playing as Luigi, after a non-castle level is completed, he takes off his hat. The same goes for Mario.
- In Super Mario 64 DS, Luigi's cap can be stolen by Klepto or a Ukiki and Luigi will be seen without his cap. He can complete the whole game without his cap, but he will be much weaker. Luigi can not get back his cap without snatching it away from the enemy that stole it or get it from Toad (who will be wearing it before giving it to Luigi) after retrieving one Star upon losing the cap.
- Upon leveling up in Mario & Luigi: Bowser's Inside Story, after the bonus upgrade is added, Luigi will hop and his hat will fall off.
- Just like with Mario, his hat disappears when he has 99 lives in New Super Mario Bros. Wii.
- The white Luma steals Luigi's hat at the end of Super Mario Galaxy 2. This is also seen with Mario.
- In Super Mario 3D Land, if Luigi is hit and reverts to his small form, he loses his cap. The same thing happens when playing as Mario, and they both regain their caps when they collect power-ups. Also, if both Luigi and Mario reach the maximum amount of lives, they will lose their caps, but have them on in their small forms.
- Luigi loses his cap when he has the maximum number of lives in New Super Mario Bros. 2, except when he is in his Silver Luigi or Raccoon Luigi forms. The same applies to Mario.
- In Luigi's Mansion: Dark Moon, when a ghost escapes to another room, Luigi takes off his cap and breathes a sigh of relief. In the ScareScraper mode, whenever he finds a warp cupboard, his hat flies in first and Luigi himself does soon after.
- Luigi's Cap is an item in Nintendogs that the player's dog can wear.
- In Animal Crossing: City Folk, there is also a Luigi Cap which the player can wear, that is called the "Lil Bro's Hat".
- Luigi's Cap color, green, when inverted turns into Waluigi's Cap color, purple.
- Baby Luigi's cap is on-purposely made larger, so it appears it was too big to wear when he was a baby.