The left-wing ERC sent the letter to Gibraltar Chief Minister Fabian Picardo on Tuesday.
In that letter, the separatist party — Catalonia's largest — offered Gibraltar support in the face of Spain's "bullying" of the UK territory.
"We most sincerely regret the improper bullying and harassment that the Spanish government is applying on the citizens whom you democratically represent," Alfred Bosch, leader of ERC in the Spanish parliament, wrote to Picardo.
Tensions between Spain and the UK have risen in recent weeks after Gibraltar began creating an artificial reef in the waters of The Rock.
Spain say the reef stops local Spanish fishermen from accessing their traditional fishing grounds and has flagged the idea of a €50 fee for anyone crossing the border between Spain and Gibraltar.
That money then would go to compensating Spanish fishermen, Spain has said.
But ERC spokesperson Bosch went on to say Spain was trying "to divert attention from its own internal political and financial scandals" in the row.
“We are convinced that the only solution to the issue of the Rock, as with the issue of Catalonia, is through dialogue, suffrage and the principle of self-determination," said Bosch in the letter.
"No question should be handled without consulting the people concerned.
"Your liberty is our liberty," the ERC letter stated.
The ERC comments are unlikely to be greeted warmly by the Spanish government in Madrid , which is staunchly anti-separatist.
The ruling Popular Party (PP) is locked in a battle with the centralist CDC government of Catalonian President Artur Mas who has called for a regional referendum on Catalonia's right to decide on the issue of independence.
The ERC won 13.7 percent if the vote in the latest regional elections in Catalonia.