The operations centre at Dakar International Airport said an Air Europa charter was expected to arrive from Fuerteventura in the Spanish Canary Islands at just after 2.30am on Thursday.
A security source, who did not want to be named, did not say how many Senegalese would be on the flight.
The issue of African migrants, thousands of whom have come ashore in the Canaries this year, has become a political issue in both countries.
Their governments have tried to keep the repatriation operations secret to foil protests.
A previous attempt by Madrid to fly illegal migrants home collapsed at the end of May after Dakar withdrew its agreement, saying its nationals were being mistreated on the flights.
Those repatriated on that occasion complained that the Spanish authorities lied to them, telling them the flight was taking them to the Spanish mainland when in fact it returned them to Senegal. They also said they were handcuffed during the flight.
Spanish police denied any mistreatment.