Gomez-Acebo & Pombo has split from its Portuguese ally Vieira de Almeida & Associados, following a review of its international strategy.
The Spanish firm could decide to launch its own office in Portugal after the split.
Gomez-Acebo's new international strategy has already seen the firm announce the opening of a London office in September.
Vieira de Almeida released a statement saying: "As a result of their cooperation, the two independent firms have an even higher regard for each other's professionalism and ethics, and they will continue to join forces whenever appropriate."
The two firms had a preferential cooperation agreement rather than an exclusive relationship.