Should I Wear Underwear Under My Swim Trunks?

When It Makes Sense

• Good fit: If you are swimming in a public pool where modesty is a priority and your swim trunks are loose, wearing a snug, moisture‑wicking brief can prevent accidental exposure.
• Good fit: When you have a medical condition such as a skin irritation or post‑surgery bandage that needs extra coverage, a thin, breathable underlayer can protect the area while you enjoy the water.

When You Should Avoid It

• Warning sign: If you are engaging in high‑intensity water sports (e.g., competitive swimming, surfing, or water polo), an extra layer can cause friction, increase drag, and reduce performance.
• Warning sign: When you are using swimwear made of chlorine‑resistant fabrics that already include a built‑in liner, adding underwear may trap chemicals and lead to irritation.

Pros and Cons

Pros

• Additional modesty and peace of mind in mixed‑gender or family environments.
• Extra barrier that can reduce direct contact with harsh chemicals like chlorine, potentially lowering skin irritation.

Cons

• Increased risk of chafing, especially if the underwear fabric does not wick moisture efficiently.
• Potential to affect the fit and hydrodynamics of the swim trunks, making movement feel bulkier.

Decision Checklist

• Will the water activity involve a lot of movement or require streamlined performance?
• Do I have a need for extra modesty or medical protection?
• Is the underwear made of quick‑dry, breathable material that won’t soak up chlorine?

Alternatives to Consider

If modesty or protection is your primary concern, look for swim trunks with an integrated mesh liner, a built‑in brief, or opt for swim briefs (speedos) that provide full coverage without an extra layer. For medical needs, consider a waterproof dress‑code bandage or a silicone skin protector designed for aquatic use.

Final Recommendation

Wear underwear under your swim trunks only when modesty, medical protection, or personal comfort outweigh the potential downsides of chafing and reduced performance. In high‑intensity or competitive settings, choose swimwear with a built‑in liner or a dedicated swim brief. When in doubt, test a quick‑dry, moisture‑wicking brief in a short swim session first, and consult a dermatologist if you have skin sensitivities.