Walking Borders, which tracks migrant deaths and provides assistance to relatives, posted on Twitter that 39 people had died, including four women and a baby.
The group’s founder Helena Maleno said the migrants had waited for over 12 hours for assistance.
A Spanish helicopter sent to the area in response to a request for help from Moroccan authorities found the dead minor and saw no other survivors, a coast guard spokeswoman said.
A Moroccan patrol boat rescued 24 people, she said, adding she did not know “how many people were on board the vessel” in total.
Contacted by AFP, there was no immediate reactions from Moroccan authorities.
The sinking happened about 160 kilometres (100 miles) southeast of Spain’s Gran Canaria.
The migrant route from West Africa to the Canary Islands across the Atlantic has become more popular in recent years as authorities have cracked down on illegal migration in the Mediterranean.
The Atlantic is notorious for strong currents that make such trips perilous.
Rescuers on Tuesday found the body of a pregnant woman on board a boat that was carrying some 50 migrants off the Canaries.
Spain is a major gateway for migrants seeking a better life in Europe.
Over 11,200 have died or disappeared since 2018 while trying to reach Spain by sea, according to a report published by Walking Borders at the end of 2022.
Last week at least 82 migrants died when the boat taking them from Libya to Italy capsized off the coast of Greece.
Authorities still have no clear idea how many people were aboard when it sank – estimates range from 400 to over 700 – making it one of the deadliest accidents involving migrants in the Mediterranean.
Police have arrested nine Egyptians on suspicion of people smuggling – one of them the captain of the boat carrying the migrants.