The last operation was in early July using the Ocean Viking vessel that is currently docked in Italy, according to Doctors Without Borders (MSF) and Sea-Watch on Thursday.
Before the Ocean Viking’s last mission, migrant rescues in the Mediterranean had been suspended as the coronavirus pandemic ravaged globally.
Announcing they will join forces, the French and German groups said they will deploy rescuers on a new ship, dubbed the Sea-Watch 4, currently docked in Spain.
Sea-Watch will provide the crew, and MSF the medical team.
"We hope to be able to depart Spain around the 10th to mid-August," MSF humanitarian affairs adviser Hassiba Hadj Sahraoui told AFP.
The mission was "essential", she said, as "currently, there is no NGO ship at sea" even as migrants escaping poverty and conflict continue attempts to cross the hazardous waters.
More than 100,000 migrants tried to cross the Mediterranean last year from north Africa with more than 1,200 dying in the attempt, according to the International Organization for Migration.
'Leaving people to drown'