The A350-900s are powered by Rolls-Royce Trent XWB-84 engines and will on an operating leases with delivery set for July and September 2016.
"We believe these aircraft will provide valuable flexibility and efficient performance for the airline’s long-haul operations," ILFC chief executive Henri Courpron said.
ILFC is a unit of American International Group (AIG). SriLankan is also acquiring another six A330 aircraft directly.
ILFC already has two A330-200s and one A340-300 on lease with SriLankan.