Underwear or no underwear? It is one of those questions that often comes up only when you are already getting ready for the beach. Swim trunks look like shorts, yet most pairs also include an inner liner—so adding another layer can feel either sensible or completely unnecessary.
The answer depends mainly on how your swim trunks are made and where you plan to wear them. Below, we’ll explain whether mesh-lined and compression-lined trunks require underwear, what happens if you wear an extra layer in the water, and whether the answer changes when swimming is not part of the plan.
Do You Need to Wear Underwear Under Swim Trunks?
Usually, no. Most modern men’s swim trunks have a built-in mesh or compression liner designed to be worn directly against the body. This inner layer provides the coverage and support you would normally expect from underwear while using materials better suited to the water.
Wearing regular underwear beneath lined swim trunks offers little practical benefit. Once wet, it can hold extra water, take longer to dry, and bunch up as you swim or walk. The additional fabric may also make the trunks feel bulky or increase rubbing during a long day at the beach.
The rule is straightforward: if your swim trunks have a comfortable, properly fitting liner, skip the underwear. If they are completely unlined, wear swim briefs or another quick-drying layer made for the water instead of ordinary cotton underwear.
Neither Mesh Nor Compression Liners Require Underwear
Swim trunks generally come with one of two liner styles, and both are built to stand in for underwear—just in slightly different ways.
Mesh lining are the more traditional option: lightweight, loose-fitting, and common in classic swim trunks. They let water pass through easily and work well for casual swimming or lounging by the pool.
Compression liners are more fitted, offering closer support and typically covering more of the upper thigh. They're common in trunks built for movement—paddleboarding, kayaking, beach volleyball, or a day that involves more than just floating around.
The details differ, but the function doesn't. Whichever liner your trunks have, it's already designed to be worn on its own, without underwear underneath.
The Downsides of Wearing Underwear with Swim Trunks
It might feel like an extra layer of security, but underwear usually works against you once you're in the water.
Regular cotton underwear absorbs water quickly and holds onto it, often staying damp long after the outer fabric of your trunks has dried. That extra moisture adds weight, and over time it can stretch out of shape or bunch up underneath the liner as you move.
Movement is really where the problem shows up. A wet, bunched-up layer creates more friction against your skin, and moisture is known to make chafing worse. If you've ever ended a beach day with unexpected irritation, a hidden layer of wet cotton is often the reason why.
This is part of why well-designed compression liner swim trunks skip the extra fabric altogether—the fitted liner is built to manage moisture and movement without adding a layer that traps water against your skin.
Comfort, Style, and Finding the Right Balance
Whether to layer up under your trunks might seem like a minor call, but it actually shapes how comfortable your entire day in the sun turns out to be.
Skip the extra layer, and you'll typically notice:
- Quicker drying time between swims
- Less friction and irritation against your skin
- Better airflow, so you're not sitting in trapped heat and moisture
- A cleaner, more tailored silhouette under your trunks
Add underwear into the mix, and you're stacking on fabric that holds onto moisture and restricts how freely you can move—not exactly ideal for a day that involves swimming, walking, or lounging in the sun. The better move is choosing well-made swim trunks with a liner built to be worn on its own, so you get the coverage without the extra bulk.
Skip the Underwear
If your swim trunks have a built-in liner—mesh or compression—you don't need underwear underneath. The liner is already designed to do that job, and it does it better than cotton ever could in the water.
The only real exceptions are unlined trunks, a liner that's worn out, or one that's been removed. Outside of those cases, wearing extra underwear just adds moisture, bulk, and unnecessary friction—and doesn't even win you anything on the hygiene front. Trust the liner, skip the extra layer, and you'll be more comfortable in and out of the water.
