Oriol Junqueras, the former Catalan vice-president, has been in pre-trial detention for the past 16 months on charges of rebellion and embezzlement for a failed bid for Catalan independence in late 2017.
His Esquerra Republicana de Cataluna is one of 45 member parties of the European Free Alliance, a pro-regionalist group that defends the right to self-determination.
“If the Spanish state wants to silence him, we will give him a voice,” said Jordi Sole, a member of the European Parliament, in a statement.
The group said it currently has a dozen MEPs sitting in different political groups that promote stronger rights and nationhood for regions such as Flanders in Belgium or Corsica in France.
Since 2014, the EU's different political families have chosen their candidate for the presidency of the European Commission, known by the German term 'Spitzenkandidat'.
Junqueras was already an MEP between 2009 and 2012.
Despite being in prison, at the end of 2017 he was elected to the Catalan regional parliament, although he did not obtain permission from the Spanish justice system to serve office.
He faces up to 25 years in prison in the ongoing trial.