Jeffy Costume
Bring the chaotic SML puppet to life with this hilarious look.
Jeffy is the wildly popular, chaotic puppet character from the SML (SuperMarioLogan) YouTube series. Fans love dressing up as him for his hilarious, instantly recognizable look that guarantees laughs at any party. The costume consists of a signature blue helmet, a yellow t-shirt with his name on it, and a pair of jeans worn under a prominent diaper.
- Difficulty
- Easy
- Time
- ~30 min
- Budget
- $30–75
Try it on
See yourself as Jeffy
One clear photo of yourself; we render you in this costume — face untouched, full body, about 20 seconds.
One render, $1. No account needed — if it fails you're not charged. Pricing.
Your photo is sent to Google's Gemini API to make the render and isn't used for training. Renders are AI-generated and labelled.
Want a different costume on this photo, or a whole group in costume? Open the Try-On Studio.
What you need for a Jeffy costume
Short on time? Buy it ready-made
Spirit Halloween sells the complete Jeffy costume — one box instead of 6 pieces.
Blue Multi-Sport Helmet
A solid, bright blue classic skate or bicycle helmet with a black chin strap.
Yellow T-Shirt
A plain, bright yellow crewneck t-shirt with 'Jeffy' written across the chest in black marker.
DIY: Write 'Jeffy' on a plain yellow t-shirt using a thick black permanent marker.
Blue Jeans
A pair of medium to dark wash straight-leg denim jeans.
Adult Diaper
A large, white disposable adult diaper worn visibly over the outside of the jeans.
Yellow Pencil
A classic yellow wooden No. 2 pencil to be placed inside one nostril or tucked behind the ear.
Black and White Sneakers
Simple black canvas low-top sneakers with white soles and laces.
Rather not assemble it? See ready-made Jeffy costumes at Spirit Halloween ↗
How to put it together
- 1
Put on the straight-leg blue jeans and the black-and-white sneakers.
- 2
Step into the adult diaper and pull it up over the outside of your jeans so it is fully visible.
- 3
Put on the yellow t-shirt. If DIYing, use a black permanent marker to write 'Jeffy' in simple print across the chest before putting it on.
- 4
Place the blue multi-sport helmet on your head and securely fasten the black chin strap.
- 5
Carefully place a yellow pencil so it rests securely under your nose/in a nostril (or safely tuck it behind your ear for comfort).
Makeup & hair
No complex hair or makeup is needed since the blue helmet covers most of your head. Keep your hair tucked flat under the helmet, and maintain a goofy, wide-eyed expression to match the puppet's personality.
Jeffy for couples, groups, kids & more
- couples
- Pair up with another SML character like Mario (Father) or Rosalina.
- group
- Gather a group to dress as the SML classroom, including Junior, Cody, and Joseph.
- kids
- Kids can easily wear this with a youth-sized blue bike helmet, a yellow tee, and a diaper over their jeans.
- Plus size
- Use a loose-fitting yellow tee and comfortable stretch-denim jeans with an adjustable diaper.
Tips
- For safety and comfort, tuck the pencil behind your ear or stick it to your upper lip with skin-safe spirit gum instead of inserting it into your nose.
- Make sure the diaper is secured tightly over your jeans so it doesn't slip down while walking.
- Practice Jeffy's signature goofy posture and catchphrases to really sell the character performance.
- Keep the helmet straps snug but comfortable, especially if wearing the costume for several hours.
Jeffy costume FAQ
- What does Jeffy wear on his head?
- Jeffy wears a classic bright blue bicycle or skate helmet with a black chin strap.
- How do you make the Jeffy shirt?
- Take a plain bright yellow t-shirt and write the name 'Jeffy' across the chest in large, black capital letters using a permanent marker.
- Why does Jeffy wear a diaper on the outside?
- It is one of the character's signature comedic design elements from the SML puppet show, worn directly over his blue jeans.
Did you know? Jeffy first appeared on the SML channel in 2016 in the video 'Mario The Babysitter!' and quickly became the channel's most famous and recognizable character.