Francken welcomed the new agreement on Twitter, pointing out that it was preceded by a similar deal with Morocco.
According to media reports, Francken and his Tunisian counterpart Adel Jarboui signed the agreement in Tunis earlier in the day.
Under the new deal, Tunisia agrees to identify migrants within 45 days. The Belgian authorities cannot keep migrants in closed centers longer than eight months, and until now Tunisian authorities' processing times often exceeded this limit.
At the moment, there are reportedly 123 undocumented Tunisians in Belgium's detention facilities.