Cathay Pacific has become the first carrier to order the Airbus A350-1000 since the twinjet underwent a substantial redesign last year.
The Hong Kong-based operator is already a customer for the A350-900.
Sixteen of these will convert to -1000s. Ten new -1000s will be ordered, giving Cathay 26 of the largest A350 version.
It will also have 20 -900s.
Cathay says it is "effectively" the first customer for the jet.
Airbus overhauled the -1000 last year to increase its range. Four other carriers had ordered the type before the redesign.
All A350-1000s will be powered exclusively by Rolls-Royce Trent XWB engines.