According to IRNA, he made the remarks while he attended a ceremony at the Cuban Embassy in Tehran to commemorate the late revolutionary leader of Cuba.
Zarif wrote a note in the embassy's memorial book condoling with Castro's family and the nation and government of Cuba.
During the ceremony, the foreign minister referred Tehrn-Havana cordial relations and called for promotion of all-out ties between the two capitals.
He stressed that Iran will always stand by the people and government of Cuba.
World will not forget Castro as the leader of a liberation movement, he added.
Zarif recalled his meeting with the Cuban leader during a recent visit to the Latin American country and said Castro was still worried about the future of Cuba and the Third World.
The late Cuban leader underlined the necessity of consolidation among the developing countries in their fight for liberation from the dominance of arrogant powers, the Iranian foreing minister said.
Neanwhile, the Cuban Ambassador in Tehran Vladimir Andres Gonzalez Quesada thanked the Iranian top officials for sending messages of condolence after Castro demise.
Iran and Cuba are committed to boost bilateral ties, the envoy added.
Cuban embassy in Iran opened a memorial book on Monday to commemorate the demise of late Cuban leader.
The Cuban embassy in a press statement declared that the memorial book will be open from Nov 28 till Dec 14.
Cuban President Raul Castro announced in a televised message that his elder brother Fidel passed away on Friday at 07:00 p.m. local time after years of illness.
Castro was a Cuban politician and revolutionary who governed the Republic of Cuba from 1959 to 1976 as prime minister and from 1976 to 2008 as president.
He had ceded power to his younger brother, Raul, in 2008 because of worsening health condition.
EghtesadOnline: Foreign Minister Mohammad Javad Zarif on Monday called demise of the late Cuban Leader Fidel Castro a 'catastrophe' for the world freedom-seeking movements, particularly those in the Latin America.