Marega has featured extensively for Portugal's FC Porto in the UEFA Champions League, scoring nine goals in three seasons.
The striker has made 25 appearances for the Mali national team and his arrival is a boost for Al-Hilal, who holds a three-point lead at the top of the Saudi Pro League.
Al-Hilal advanced to the 2021 AFC Champions League Round of 16 as one of the three best runners-up, finishing second in Group A behind Tajikistan's FC Istiklol.
The Riyadh-based club will face Iran's Esteghlal in the Round of 16, which is scheduled for September.