The Spanish PR independent replaced rival Llorente & Cuenca after a two-month pitch process, which included two other unnamed agencies.
Llorente & Cuenca had handled PR for GO before the merger.
Inforpress founder Nuria Vilalonga said the agency's one-year brief centres around press relations work for the airline, which floated nearly three years ago and was founded by easyGroup boss Stelios Haji-Ioannou.
The agency will specifically work to promote easyJet's fares, scheduled flights and its 'lifestyle solutions' for travellers.
'We are going to do everything related to the company's offers and characteristics, which will include interviews, trips and other events for the press to get to know the company,' said Vilalonga.
Prior to last year's merger with GO, which was formerly owned by British Airways, easyJet had handled all PR for the Spanish and Portuguese region in-house.
News of the airline's agency appointment coincides with its expansion plans in Spain and Portugal.
The carrier has been adding flights to the region and now serves seven destinations in Spain and one in Portugal, the Saro tourist resort.
Vilalonga said that Inforpress will also work to differentiate easyJet from Spanish airlines Iberia, Air Europa and Spanair, which have been planning their own low-cost offers to compete with the carrier.
The four-strong Inforpress team will report to easyJet marketing manager in Spain and Portugal Cristina Bernabe.
Launched in the late 1980s by Vilalonga, Inforpress currently has offices in Barcelona, Madrid, Valencia and Lisbon.