Figuring out exactly what to wear under a gi is usually the first thing on a beginner's mind before they step onto the mats. It feels like a small detail, but if you get it wrong, you'll definitely notice it ten minutes into a hard training session. Whether you're starting Brazilian Jiu-Jitsu, Judo, or Karate, the layers you put on beneath that heavy cotton uniform can make the difference between a great workout and a miserable, chafing-filled hour.
Most people show up to their first class with just a random t-shirt and some gym shorts, which is totally fine for day one. But as you stick with it, you'll realize that cotton isn't exactly your best friend when things get sweaty. The goal is to find gear that stays in place, protects your skin, and doesn't make you feel like you're wearing a wet towel by the end of the night.
The Case for Rash Guards
If you ask any long-term practitioner what they prefer, nine times out of ten they're going to tell you to get a rash guard. These are those skin-tight, stretchy shirts made of polyester and spandex. They're popular for a reason. Unlike a standard cotton tee, a rash guard is designed to wick moisture away from your body.
When you're training in a gi, you're basically wearing a heavy canvas suit. You're going to sweat—a lot. A cotton t-shirt absorbs that sweat, becomes heavy, and starts to sag. Even worse, it can get caught in your partner's fingers or toes during a scramble. A rash guard stays glued to your skin, so there's no extra fabric flying around.
Another big perk is protection. The fabric of a gi is pretty abrasive. If you go "commando" under the jacket or just wear a thin shirt, you'll likely end up with gi burn on your elbows, shoulders, and neck. A long-sleeve rash guard acts like a second skin, preventing the gi from grinding directly against your flesh. Plus, from a hygiene standpoint, it creates a barrier between your skin and the mat, which helps keep nasty stuff like staph or ringworm at bay.
Can You Just Wear a T-Shirt?
Look, if you're just starting out, don't feel like you have to go out and spend a fortune on specialized gear. A t-shirt is perfectly acceptable for your first few weeks. However, there are a few things to keep in mind. Try to pick a shirt that fits relatively tight. A baggy shirt will end up bunched up around your armpits or, even worse, the collar will get stretched out until it's unwearable.
Also, be prepared for that shirt to be ruined eventually. The constant pulling and friction from the gi jacket will eventually tear the seams or stretch the neck into a weird shape. If you're okay with your old gym shirts becoming "mat clothes," then go for it. Just know that once you try a proper compression shirt or rash guard, you probably won't want to go back to cotton.
What About the Bottom Half?
Deciding what to wear under your gi pants is arguably more important than the top. You need something that provides a full range of motion but keeps everything "secure." Most people opt for compression shorts or spats (which are just full-length leggings for grappling).
Compression shorts are great because they prevent thigh chafing. When you're moving through various guards or throwing someone in Judo, your legs are doing a ton of work. If you're just wearing loose boxers, you're going to deal with some serious discomfort. Most practitioners prefer compression shorts that have a pocket for a cup, though whether or not you should wear a cup is a whole different debate in the martial arts world.
Spats are another fantastic option, especially in the winter or if you have sensitive skin. Just like the long-sleeve rash guard, spats protect your knees and shins from the rough interior of the gi pants. They also keep your legs from sticking to the mat when you're sweating. It sounds a bit weird, but having that layer of fabric makes movement a lot smoother.
Choosing the Right Underwear
This is a bit of a "taboo" topic that doesn't get talked about enough, but it's practical. Avoid loose-fitting boxers at all costs. They bunch up, they offer zero support, and they hold onto moisture like a sponge.
Most people find that athletic briefs or compression-style underwear are the way to go. You want something that stays put. There is nothing more distracting during a live roll or a sparring session than having to constantly adjust your gear because things are shifting around. Keep it simple, keep it tight, and make sure it's a synthetic material that dries quickly.
Gender-Specific Considerations
For women, figuring out what to wear under a gi involves a few extra steps. A high-quality sports bra is non-negotiable. You'll want something with a high level of support because there is a lot of movement, jumping, and occasionally taking falls.
Many women also prefer wearing a full rash guard over their sports bra rather than just a tank top. This prevents the gi jacket from rubbing against the ribs and chest, which can get pretty uncomfortable during long sessions. Also, high-waisted leggings or spats are often preferred over shorts to ensure everything stays covered regardless of what position you end up in on the mats.
Hygiene and Etiquette
While we're talking about what to wear, we have to talk about cleanliness. The unwritten rule of the gym is that your under-gi gear must be just as clean as your gi itself. Never, ever reuse a rash guard or compression shorts from a previous session without washing them. Bacteria thrives in the warm, damp environment of a gi, and wearing dirty gear is a fast track to skin infections for you and your training partners.
It's also worth checking with your specific school or dojo. Some traditional schools have very strict rules. They might require a plain white t-shirt or a specific color of rash guard under the gi. Others are totally laid back and don't care if you're wearing a neon-colored superhero rash guard. Just take a look at what the upper belts are wearing, or just ask the instructor after your first class.
Why Material Matters
You'll see "moisture-wicking" on almost every piece of athletic gear these days, but it's actually a big deal for martial arts. When you're trapped in a heavy gi, your body temperature spikes. If your base layer is soaking up that sweat and holding it against your skin, you're going to overheat much faster.
Synthetic blends like polyester, nylon, and elastane are the gold standard. They allow sweat to evaporate more efficiently and they don't get heavy when wet. They also have "four-way stretch," meaning they move with your body rather than resisting it. When you're trying to kick, shrimp, or throw, you don't want your clothes fighting against you.
Conclusion
At the end of the day, finding the perfect setup for what to wear under a gi is a bit of a trial-and-error process. Some people love the feeling of full-length spats and long sleeves because they feel protected and "slick" on the mats. Others find that too hot and prefer just compression shorts and a short-sleeve rash guard.
If you're unsure, start with a basic short-sleeve rash guard and some compression shorts. It's the most versatile combo and works for almost any grappling or striking art. As you spend more time on the mats, you'll figure out what feels best for your body type and your style of movement. Just remember: stay away from loose cotton, keep it clean, and make sure you're comfortable enough to focus on the technique rather than your clothes. Once you get your gear dialed in, you won't even think about it—and that's exactly the goal.