Some 128 people were reported to have previously been on the vessel when emergency responders were called out on Wednesday.
The rescuers reached the boat – which had left The Gambia 27 days earlier – about midnight, and brought survivors on shore.
Helena Maleno, founder of NGO Walking Borders, said on social media that survivors reported three women and a baby were among those who died during the crossing.
Maritime Rescue told Reuters that five bodies were on board the vessel when rescuers reached it.
A statement from Maritime Rescue to Sky News said: “Yesterday afternoon we located a migrants boat departed from Gambia on 7 August that was in high difficulty off the southern coast of Gran Canaria.
“We immediately sent assistance and rescuers, which reached the boat around midnight.
“At dawn this morning, we confirmed approximately 80 deaths, out of a total of 128 passengers. The survivors were taken to the Arguineguín pier and are being cared for by local authorities.”
The bodies of many of those who died could not be recovered because of adverse weather conditions, Maritime Rescue said.
Fernando Clavijo, the Canary Islands regional leader, said the tragedy underscored the need for a state policy on migration, especially after a deadly migrant surge in Spain’s North African enclave of Ceuta in July.
He said on X: “It is intolerable that lives continue to be lost. The Canaries cannot get used to or accept as normal the fact that the Atlantic continues to be a death trap.”
The Atlantic route from West Africa to the Canary Islands is one of the world’s deadliest migration crossings.
The journey is attempted by thousands of migrants each year, who try to reach the Spanish territory in overcrowded boats, despite the risks posed by rough seas, strong currents and long distances.
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