What to Wear Under Swimwear: Swim Briefs vs Trunks

Quick answer

Most men should not wear everyday underwear under swimwear. Swim trunks and swim briefs are designed to be worn alone, and most have a built-in liner or pouch that handles support. If your swimsuit has no liner and you want support, wear a swim-specific brief underneath rather than cotton boxers or briefs, because cotton holds water and chafes.

The short answer to whether you need underwear under swim trunks is usually no, but the reason matters more than the rule. Swimwear is built to get wet, drain, and dry, while everyday underwear is built for dry land. Layering the two is what causes the bunching, sagging, and slow-drying discomfort people complain about at the pool.

The real decision is not underwear versus no underwear. It is which swim style gives you the support and coverage you actually want: a swim brief with a lining or pouch, or a trunk with a liner. Below is a step-by-step way to choose, plus what to check before you buy.

Why swimwear is made to be worn alone

Swim fabric is engineered to shed water and dry quickly. Cotton underwear does the opposite: it soaks up water, gets heavy, and stays damp against your skin. That is the main reason you should not wear regular underwear under a swimsuit. If you want extra support, the right layer is a swim-specific piece, not your daily boxers or briefs.

Many swimsuits also include a built-in liner or an internal pouch. That liner is doing the job underwear would do, but in a material that drains and dries. Adding a second layer on top of a liner usually just creates bulk and friction.

• Check the label or product page for a liner or pouch before deciding to add anything underneath.
• If you are unsure, wear the suit alone for a short swim first and see how it feels.

Swim brief vs trunk: what actually changes

A swim brief sits higher on the leg and offers less coverage, which makes it the choice for swimming laps, open water, or anyone who wants minimal drag and maximum freedom of movement. A trunk looks closer to a short, with more coverage on the thigh, and tends to suit casual pool days, beach walks, and mixed settings where you want a more covered look.

The difference is not just style. Coverage affects how the suit stays in place when you move. Briefs rely more on a snug fit to stay put, so the waist and leg openings need to be right. Trunks have more fabric to hold them in position, which can be more forgiving if you are between sizes.

• Choose a brief for lap swimming and minimal coverage, a trunk for more coverage and everyday beach use.
• Whichever you pick, the waistband should sit flat without digging in.

How to handle support if you want it

If your swimsuit has no liner and you want support, look for a swim brief designed for that purpose, or a swim style with a built-in pouch. Rounderbum's H2O Padded + Package Swim Brief uses Smart Cup Tech, and the H2O WAVEFORM Padded + Package Swim Brief is part of the same re-engineered H2O line. These are swim pieces, so they drain and dry like swimwear rather than behaving like cotton underwear.

If you are swimming laps or spending hours in the water, prioritize a secure fit over extra padding. Padding and pouch features are more about appearance and confidence, while fit is what keeps the suit comfortable when you move.

• Never substitute cotton briefs for a swim liner; they hold water and stay heavy.
• If a suit has a liner, try it alone before adding a second layer.

Fit checks that prevent most swimwear problems

Most complaints about swimwear, from riding up to feeling exposed, come down to fit rather than the style itself. A brief that is too loose will shift when you swim. A trunk that is too tight will dig in at the waist and legs. The fix is usually a size or style change, not an extra layer of underwear.

Try the suit on and move through a few motions: squat, raise your knees, and rotate your hips. If the waistband rolls or the leg openings gap, that is your signal to adjust size or switch between a brief and a trunk. Rounderbum's swim line is available in Color and Size options, so you can dial in the fit rather than guessing.

• Test the suit with movement, not just by standing still in front of a mirror.
• If the leg openings gape, size down or move to a trunk for more coverage.

How to choose what to wear under swimwear

1. Check whether your swimsuit has a liner or pouch: Look at the product page or tag. If a liner or pouch is present, it is already doing the job of underwear. If there is a liner, wear the suit alone. Adding underwear on top is the most common mistake.
2. Decide between a swim brief and a trunk: Pick a brief for lap swimming and minimal coverage, or a trunk for more thigh coverage and casual wear. Match the style to the activity first, then worry about color.
3. Skip cotton underwear entirely: Cotton absorbs water, gets heavy, and stays damp. If you want support without a liner, use a swim-specific brief instead. Any layer under swimwear should be made for water, not for dry land.
4. Test the fit with movement: Squat, raise your knees, and rotate your hips. The waistband should stay flat and the leg openings should not gape. If it shifts or digs in, change size or switch between a brief and a trunk before adding layers.
5. Rinse and dry after every swim: Chlorine and salt shorten the life of swim fabric. Rinse in cool water and air dry away from direct heat. Good care keeps the liner and pouch in shape for the next swim.

Swim styles worth knowing

• H2O Padded + Package Swim Brief: Part of the H2O swimwear line, re-engineered for a better fit and appearance, and it features Smart Cup Tech. A swim-specific brief with built-in support, so you do not need to add underwear underneath.
• H2O WAVEFORM Padded + Package Swim Brief: Part of the H2O line, which translates Rounderbum's technology into swimwear and was re-engineered for a better fit. A bolder, brighter brief option for swimmers who want a swim brief with pouch features rather than a plain liner.
• H2O Lift Swim Trunk: Based on Rounderbum's LIFT TECH and designed for versatility from beach to bar. A trunk option for men who want more coverage than a brief but still want the brand's support technology.

For most men, the answer to what to wear under swimwear is nothing, as long as the suit has a liner or pouch and fits well. Choose a swim brief for swimming and minimal coverage, or a trunk for more coverage, and let the swimwear do the job it was designed for.