What to Wear Running: A Complete Guide to Running Underwear

A cotton brief feels fine for the first mile and miserable by the fifth, because it soaks up sweat and holds it against your skin. The fix is simple: pick underwear built for training, not for the drawer you’ve had since college. Fabric, fit, and seams are the three things that decide whether a long run ends in comfort or in chafing you’ll feel for days.

What Fabric Works Best For Running?

Polyester, nylon, spandex, and merino-based performance fabrics pull sweat away from your skin and dry fast, which is why running guides consistently recommend them over cotton. Adidas groups this type of underwear under its moisture-wicking training and competition category, built for exactly that purpose.

Cotton is the one material to leave at home. It holds moisture instead of moving it, so a sweaty run leaves you damp, heavy, and far more prone to irritation. Merino blends are the exception worth knowing about: wool handles sweat and temperature swings well, though it costs more and dries slower than synthetics.

• Synthetics (polyester, nylon, spandex): fastest drying, cheapest, best for hot-weather runs.
• Merino blends: better temperature regulation for cold runs, pricier, slower to dry.
• Cotton: skip it — it retains sweat and multiplies friction.

How Tight Should Running Underwear Be?

Snug enough to stay put, loose enough to move freely. That’s the whole test. Underwear that shifts around during a run causes rubbing and wedgies; underwear that digs in restricts your stride and irritates skin.

Look for full-coverage cuts rather than minimal ones. Briefs and boxer briefs that stay in place through a full stride beat anything you have to adjust mid-run. If a pair rides up or bunches within the first few minutes, it will only get worse over an hour.

Seams are where most chafing starts. Seamless, tag-free construction removes the two biggest friction sources — stitched seams and scratchy labels — and it’s the single upgrade runners notice most on long, sweaty efforts. If you’re weighing specific options, our tested roundup of briefs for running covers fit and fabric pairings that hold up past the ten-mile mark.

Should You Wear Underwear Under Running Tights?

Usually not. Running tights are designed to be worn on their own, and ASICS notes that adding underwear underneath is optional rather than required. Tights worn alone mean one less layer to bunch, rub, or hold sweat.

That said, plenty of runners prefer the extra coverage and wear underwear anyway — a thin, seamless synthetic pair adds almost nothing under a tight. The ASICS guide to running tights also stresses fit above all: tights should sit close like a second skin, stay opaque, and match the weather, with sweat-wicking ventilation for warm runs and thicker coverage for cold ones.

What Are The Most Common Mistakes?

The biggest one is wearing cotton, followed closely by ignoring fit and seam placement. Both turn a normal run into a slow, uncomfortable grind.

• Choosing cotton because it’s what’s in the drawer — it holds sweat and increases irritation.
• Going too loose, which lets fabric slide and rub with every stride.
• Going too tight, which restricts movement and creates pressure points.
• Ignoring seams and tags, the top cause of chafing on longer runs.
• Assuming you need underwear under tights when the tights are built to be worn alone.

Runner’s World’s running-underwear roundups for both 2024 and 2025 land on the same short list of traits: moisture-wicking fabric, seamless or tag-free build, and a close fit. Runderwear markets its line specifically as anti-chafing and breathable, while The category is consistent, and so is the advice.

Before you buy, check the moisture-wicking training underwear category for the attribute labels — moisture-wicking, breathable, quick-dry — and treat them as the checklist to shop against.