The 25-year-old, who has spent the past two seasons on loan at Argentinian side Atletico Tigre, will join the Barcelona-based club on a four-year deal.
A statement on www.rcdespanyol.es read: 'Roman Fernando Martinez will be the fourth signing for the Blanc-i-blaus this season, after an agreement in principle was reached in the last few hours between Espanyol and Argentinian club Arsenal de Sarandi.
'The negotiations, which were carried out in Barcelona until the early hours of the morning, will allow Roman Martinez to sign (with the club) for the next four seasons.'
Martinez has been one of the revelations of this season's Apertura league in Argentina, where he has helped newly promoted Tigre to second place.
His arrival follows those of Argentinian goalkeeper Cristian Alvarez, defender Gregory Beranger from Numancia and Jose Callejon from Real Madrid.