Since joining the club in 2019, the 25-year-old made 278 appearances for Arsenal in all competitions, scoring 62 goals, helping them win the Premier League last season, the FA Cup in 2020, and the FA Community Shield in 2023.
"The board of directors of Al Hilal Club, headed by His Highness Prince Nawaf bin Saad, signed Brazilian player Gabriel Martinelli from Arsenal FC, to represent Al Hilal first football team for four years," Al Hilal said on their website.
"Everyone at Arsenal would like to thank Gabi for his contribution to the club and wish him and his family well for the future," the English champions said on their website.
Martinelli followed Premier League players Crysencio Summerville and Ollie Watkins, who joined the Saudi club this year from West Ham United and Aston Villa.