Aukera Guztiak (AG) was banned by Spain's Supreme Court on Saturday from fielding candidates in the Basque parliamentary election due to its purported ties to armed separatist group ETA.
The court ruled that AG's campaign was an attempt by ETA and outlawed party Batasuna to subvert laws banning them from running for office.
AG has denied ties to either group and Batasuna has always denied being part of ETA.
"Aukera Guztiak will appeal the ruling from the Supreme Court, not because we have much hope but in order to exhaust all the legal avenues and to confront all the anti-democratic measures taken against Aukera Guztiak," party spokeswoman Marije Rodriguez de Lera told reporters on Sunday.
The appeal goes to Spain's Constitutional Court, which is expected to make a decision before formal campaigning in the Basque region begins on 1 April.
ETA has killed more than 800 people since 1968 in demand of an independent Basque homeland in northern Spain and southwestern France, though it has not staged a fatal attack nearly two years amid a heavy police crackdown.