The extra A320 is set to begin operating from July 27.
The budget airline will also upgrade two A319s to larger A320s, which will “significantly increase” its seat capacity at Liverpool.
More than 3,000 easyJet flights are scheduled from the north-west airport next summer, providing more than one million seats between June and September.
The announcement came a day after easyJet placed its summer 2018 flights programme on sale.
Ali Gayward, easyJet’s UK commercial manager, said: “We have been committed to Liverpool airport for nearly 20 years and we are proud to have carried more than 40 million passengers in that time.
“We are delighted to demonstrate our commitment to the north-west market with this growth, which will enable us to offer more seats to more destinations from next summer.”
The news follows easyJet adding four more airline partners to its recently launched long-haul feeder flights network. The carrier’s Worldwide by easyJet platform will now offer connecting services with Italian airline Neos, Guernsey-based carrier Aurigny and the French duo of La Compagnie and Corsair, by the end of the year.
EasyJet said the collaboration would open up 30 regional and long-haul destinations to its network.
The service was launched last month with Norwegian and WestJet.
