For standard revenue passengers in 2026, United Airlines does not enforce a formal dress code for Domestic First Class or international United Polaris business class. You are permitted to wear casual attire, including jeans, t-shirts, and sneakers, provided the clothing is clean, not overly revealing, and lacks offensive graphics. However, a strict dress code still applies to "Pass Riders" (employees or their guests traveling on standby). For these travelers, attire must be "well-groomed, neat, and in good taste." Prohibited items for pass riders include flip-flops, beachwear, clothing with holes, or excessively short skirts/shorts. For paying customers, while "athleisure" is the 2026 norm, many travelers still opt for "smart casual" to match the premium environment of the United Polaris Lounges. Regardless of your ticket, footwear is mandatory at all times for safety and hygiene, and extremely malodorous or disruptive clothing can still lead to a boarding denial under general "Conditions of Carriage."