There isn't a dress code for first class specifically. Very few airlines publish or enforce any kind of special dress code for revenue/regular passengers, so if you're traveling on a paid ticket or an award ticket, you can relax. Note, however, that many airlines do have a general dress code.

United Airlines does not have a strict, written dress code for "revenue" (paying) passengers in First Class, though "Smart Casual" is the widely accepted standard. You are generally permitted to wear neat jeans, sneakers, and clean T-shirts. However, the airline reserves the right to deny boarding to anyone wearing clothing that is overtly offensive, excessively dirty, or "inappropriately revealing" (such as swimwear or sleepwear). In 2026, the rules are significantly stricter for "Pass Riders" (airline employees or their families traveling on standby); they are explicitly prohibited from wearing ripped jeans, shorts more than three inches above the knee, flip-flops, or gym wear. For paying passengers, while you won't be kicked out for wearing a hoodie, most travelers in premium cabins like United Polaris opt for a more polished look to match the "luxury" environment of the Polaris lounges, which feature sit-down dining and spa services that favor a business-casual aesthetic.