Ayari, a 24-year-old Tunisian national, is the ninth person to be charged over the bombings at Brussels airport and on the city's metro in March 2016.He was jailed in April, along with Abdeslam, for opening fire on police officers who tried to arrest the pair when they were holed up in a Brussels flat.
On Thursday he was charged with "participation in the activities of a terrorist organisation", the federal prosecutor's office said.
Responsibility for the Brussels attacks, which took place after Abdeslam and Ayari were arrested, was claimed by the Islamic State armed group, with whom Ayari is believed to have fought in Syria.
Investigators suspect Ayari took part in a plot to attack Amsterdam-Schipol airport by the same cell believed to have carried out both the Paris and Brussels attacks.
Ayari's indictment opens the way for a similar one for Abdeslam, a Belgian-born Frenchman, in connection with the Brussels bombings, according to Belgian media.