Should I Wear Underwear Under My Swim Trunks?

When It Makes Sense

• Good fit: When you are swimming in a public pool or beach where the water may be chlorinated or salty, a thin, moisture‑wicking brief can provide an extra layer of protection against skin irritation and help keep the swim trunks from clinging to sensitive areas.
• Good fit: If you plan to engage in mixed water‑land activities—such as beach volleyball, paddle‑boarding, or a boat outing—underwear can give you a familiar, secure feel that reduces the need to constantly readjust the trunks, especially if the trunks have a looser cut.

When You Should Avoid It

• Warning sign: When the swim trunks are made from a stretchy, form‑fitting fabric (e.g., Lycra or spandex blends), adding underwear can create bulk, increase friction, and lead to chafing, especially during prolonged swimming.
• Warning sign: If you are participating in competitive swimming or any activity where drag and hydrodynamics matter, any extra layer—including underwear—can slow you down and is generally discouraged by coaches and swim regulations.

Pros and Cons

Pros

• Provides an additional hygienic barrier, reducing direct contact between the trunk material and intimate skin, which can be helpful in heavily chlorinated or salty water.
• Offers extra support and reduces the need for frequent readjustments, which can be convenient during mixed‑activity days at the beach.

Cons

• Potential for increased friction and chafing, especially if the underwear material does not wick moisture well or if the trunks are tight.
• May affect the fit and aesthetic of the swim trunks, causing them to ride up or appear baggy, which can be uncomfortable or unwanted.

Decision Checklist

• Will I be in a setting where extra hygiene protection is valuable (e.g., public pool, salty ocean water)?
• Do my swim trunks have a relaxed cut and breathable fabric that can accommodate an extra layer without causing chafing?
• Am I engaging in any activity where reduced drag or maximum mobility is critical (e.g., competitive swimming, fast water sports)?

Alternatives to Consider

If you want the benefits of an extra barrier without the downsides of underwear, consider swim‑specific undergarments such as compression briefs or swim‑short liners that are designed to be worn beneath trunks. Alternatively, choose swim trunks with a built‑in inner mesh liner, which provides support and hygiene without the need for a separate underwear layer.

Final Recommendation

For most casual beach or pool outings, wearing a thin, moisture‑wicking brief underneath swim trunks can improve comfort and hygiene, provided the trunks are not overly tight. In contrast, for activities that demand optimal fit, reduced drag, or where chafing is a concern, it is wiser to skip underwear or select swim‑specific liners. When in doubt, test the combination in a low‑stakes setting first, and consult a swim coach or dermatologist if you experience persistent irritation.