Thermal Underwear & Base Layers

Thermal Underwear & Base Layers for Cold Weather

Thermal underwear and base layers for hiking, fishing, travel, winter sports and everyday outdoor use. This collection includes merino wool, merino blends and technical synthetic base layers, with tops, leggings, pants and selected complete sets for different temperatures and activity levels.

How to Choose Thermal Underwear

• Merino wool: a strong choice for warmth, comfort and moisture management during cold weather and lower- to moderate-intensity activity.
• Technical synthetics: quick-drying fabrics are well suited to hiking, sport and higher-output use where fast moisture transfer matters.
• Fabric weight: compare GSM, construction and intended season instead of choosing only by material name.
• Fit: a base layer should sit close to the body without restricting movement so it can manage moisture effectively.

Merino or Synthetic Base Layer?

Merino base layers are popular for warmth, softness and odor management, while technical synthetic layers usually dry faster and work well during more intensive activity. For many users, the best choice depends on temperature, trip length and how much you expect to sweat.

Men's and Women's Thermal Underwear

For gender-specific fits and size ranges, browse men's thermal underwear and base layers and women's thermal underwear and base layers.

Build a Layering System

Thermal underwear is the first layer. Add a fleece jacket or mid-layer for insulation, then choose a softshell jacket, waterproof jacket or winter insulated jacket depending on weather and activity.

Learn More About Base Layers

Read our thermal base layer guide for a more detailed comparison of merino, synthetic fabrics, warmth levels and layering for men and women.