The 35-year-old, who announced his international retirement following this year's World Cup, departed Jeddah-based club Al Ahli after a decorated three-year stint.
"His left foot made history and our ambition is writing the next chapter. Riyad Mahrez in Shamal colours," the Qatari club posted on X.
Mahrez made his professional breakthrough at French side Le Havre before he joined Leicester City in 2014, where he became a central figure in the English club's fairytale Premier League triumph in 2016.
The 2016 African Footballer of the Year later featured for Manchester City, winning four Premier League titles and the Champions League before moving to Al Ahli.
Mahrez helped Al Ahli clinch back-to-back AFC Champions League Elite titles in 2025 and 2026.
The winger, who was born in France, played a key role in Algeria's successful campaign to win the 2019 Africa Cup of Nations and helped the team reach the last 32 at this year's World Cup.
He scored 40 times in 119 international appearances.
Shamal are fifth in the Stars League standings with eight points from five games.