This plane has 5 sections, with 2 Business class sections at the front of the plane followed by a Premium Economy class section and 2 Economy class sections. Toilet suitable for passengers with reduced mobility, emergency exit rows and extra-legroom seats are located at the front of the last Economy class section.

The seating arrangement on an Airbus A350-900 is designed around its "extra-wide-body" fuselage, which typically allows for a very comfortable 3-3-3 layout in Economy Class. This 9-abreast configuration is standard for major 2026 carriers like Delta, Singapore Airlines, and Qatar Airways, offering seats that are roughly 18 inches wide—noticeably more spacious than the 787's tighter 9-abreast setup. In Premium Economy, airlines usually opt for a 2-4-2 or 2-3-2 arrangement, providing wider seats and dedicated leg rests. Business Class is almost universally configured in a 1-2-1 "staggered" or "herringbone" layout, ensuring that every passenger has direct aisle access and a seat that converts into a fully flat bed. Because the A350 has nearly vertical sidewalls and the largest overhead bins on the market, the cabin feels exceptionally airy and "open." Some low-cost or high-density carriers may push the Economy cabin to a 10-abreast (3-4-3) configuration, but for the vast majority of international long-haul travelers, the 3-3-3 arrangement remains the "gold standard" for comfort on this advanced, fuel-efficient aircraft.