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Man wearing Sir W. Cassius athletic tee, holding a wooden baseball bat wearing a cap and casual outfit on a beige background
Black and shadow Sir W. Merino t-shirts on hangers
Man wearitn Sir W. Merino Cassius Black Tee and holding a baseball bat on a green mat with a beige background
Black Sir W. Merino t-shirt on a wooden hanger
Black Sir W. shirt with 'Sir W' branding
Man wearing a black Sir W. Merino t-shirt and shorts on a beige background
Man wearing a black Sir W. Merino t-shirt and shorts on a beige background
Man wearing a black Sir W. Merino Cassius t-shirt and shorts on a beige background
Man in black Sir W. Merino t-shirt and white pants holding a golf club against a beige background
Sir W. Merino Black shirt on a wooden hanger against a beige background
Man wearing Sir W. Merino cassius shadow grey tee and white pants with navy Sir W. cap.
Man wearing Sir W. Merino cassius shadow grey tee and white pants with navy Sir W. cap while holding baseball bat
Man wearing Sir W. Merino cassius shadow grey tee from the back
Man wearing Sir W. Merino cassius shadow grey tee and black shorts
Sir W. Merino cassius shadow grey tee, with white pants and navy cap.
Sir W. Merino cassius shadow grey t-shirt hanging on a wooden hanger against a beige background
Sir W. Merino cassius shadow grey with white ram logo side profile
Sir W. Merino cassius shadow grey tee and white pants with coffee in hand.
Sir W. Merino cassius shadow grey tee hanging with logo
Shop  /  Tees  /  The Cassius- Merino Athletic Tee

The Cassius- Merino Athletic Tee

The performance tee that wicks like polyester — without the plastic, the smell, or the microplastic shedding.

Sale price$90 USD
5×
Wear It Five Times Between washes
℃
Temp-Regulating Cool in heat, warm in cold
∅
Odour-Resistant Naturally antibacterial
♺
Zero Plastic 100% biodegradable
Size:
Color:Black
Raglan sleeves with full shoulder mobility. Relaxed through the body. Runs one size larger in the chest than The Bruce — size down for a slimmer fit.
Free US Shipping
3–5 business days
30-Day Returns
Free in the US, no questions
Australian Merino
Five generations, since 1880
The Math

A merino gym tee that lasts longer than the polyester one — without the plastic.

The Cassius Cotton Tee Polyester Tee
Wears per wash 3–5 1 1
Years of regular wear 4–6 1–2 1
Holds odour? No Yes Yes (worse)
Hot & cold weather? Both Cool only Neither
Microplastics per wash None None 100,000+
Cost per year (5 yrs) $18 $20 $32

The performance tee that's actually clothing — not a plastic bottle.

100% superfine Australian Merino. Raglan sleeve construction for full range of motion. Lightweight fabric. Crew neck. Designed to wear as a base layer, on its own, or under a button-up.
Raglan sleeves with full shoulder mobility. Relaxed through the body. Runs one size larger in the chest than The Bruce — size down for a slimmer fit.

View full size guide →
Wash cold on wool cycle. Lay flat to dry. Refresh between wears by hanging overnight — most owners go 3–5 workouts between washes without ever smelling.

Full care guide →
Named for Cassius, our champion ram in 1975, who was a prized ram of Sir W. Some sheep are just better. So is this tee.
Black Sir W. Merino t-shirt on a wooden hanger
The Cassius- Merino Athletic Tee Sale price$90 USD
The Starter Set

Want all three? Save $35.

One Bruce in Caramel, one George in Royal Blue, one Cassius in Shadow — the wardrobe rotation, at $290 for the set.

Shop The Set →