The agreement raises to 19 the number of A350s due to the Oneworld carrier.
Finnair says it is intending to withdraw its Airbus A340s, of which it has seven, by the end of 2017 as the A350s enter the fleet.
As part of the agreement, it says, Airbus has agreed to acquire four of its A340-300s in 2016-17.
Finnair was one of the first operators to order the A350, initially agreeing to take the type in 2006. All A350s are available only with Rolls-Royce Trent XWB engines.
Chief executive Pekka Vauramo says the A350 will be "essential" to its long-haul development strategy, and that firming the options "demonstrates our commitment to growth".
He adds that the A340 swap with Airbus "ensures a smooth transition" between the old and new aircraft, "mitigating potential business continuity risks related to fleet renewals".
Under its long-haul modernisation programme, Finnair will take its first four A350s in the second half of next year. Another seven will be delivered over 2016-17 with the remaining eight arriving over 2018-23.
Finnair also uses eight A330s for long-haul operations. The carrier, which has arranged sales and leaseback for four A330s and two A350s, says it is "evaluating various financing options" for the other A350 deliveries.
It adds that the combination of the A340 switch and the previously-disclosed leasebacks are "not significant" to the airline's operating profit outlook for 2014 and 2015.
(David Kaminski-Morrow - FlightGlobal News)