The A350-800 will seat 270 passengers in a 3-class with a 9-abreast layout. It will have a range of 15,400 km (8,300 nmi). It is designed to compete with the Boeing 787-9 and to directly replace the Airbus A330-200. In January 2010 Airbus announced that the -800 would be developed as a simple shrink of the -900, incorporating minor changes to the systems and structure and share more hardware with the -900 rather than as an optimised variant as was previously planned. This increased commonality will allow a higher maximum takeoff weight, which will increase the range (or payload) of the A350-800 compared to initial plans. The change will increase fuel burn by "a few per cent", according to the programme's marketing head, Sophie Pendaries.

The -800's fuselage is 10 frames shorter (six forward and four aft) than the -900 aircraft. The baseline -800 will be offered with an MTOW of 248 t (550,000 lb), MLW of 190 t (420,000 lb), MZFW of 178 t (390,000 lb), and 330 kN (74,000 lbf) thrust engines. An optional 11-tonne (24,000 lb) increase in MTOW, to 259 t (570,000 lb) with a corresponding increase of MZFW to 181 t (400,000 lb), MLW to 193 t (430,000 lb), and a higher thrust 370 kN (83,000 lbf) engine (common with -900 engine thrust) was announced by Airbus in April 2010 to be made available for customers as an option. While the increased weights compensate for the increased empty weight of the aircraft and associated minor fuel burn penalty due to maintaining commonality with -900, it also resulted in an increase in the aircraft maximum structural payload capability by 3 t (6,600 lb), or 459 km (248 nmi) of additional range.