The 26-year-old arrived at St James' Park from Nice in August 2019, and he made 124 appearances for the club in all competitions, scoring 13 goals and registering 21 assists in the process.
Saint-Maximin made 31 appearances for the Magpies last term, but he only scored once and provided five assists, and there has been a host of speculation surrounding his future this summer.
A number of Saudi Arabian clubs are believed to have shown an interest in his services, but it has now been confirmed that he has made the move to Al-Ahli.
"Allan Saint-Maximin has joined Saudi Pro League side Al-Ahli for an undisclosed fee, bringing his four-year spell at Newcastle United to an end," read a statement on Newcastle's official website.
"Four years ago, I wore the Newcastle jersey for the first time. At that moment, I hadn't realised that not only was I about to become a Newcastle player, but I was also about to become a Geordie," Saint-Maximin posted on his official Twitter account on Saturday.
"You might think 'nice goodbye message' but the sentiment runs deeper than any words I could say, from the very first months that I met the people of this city, I truly understood them and embraced their unwavering passion, which is why it was so important for me to give my all and if this meant to play injured or even not making statistics, I didn't give a sh** as long as it would help my team.
"Believe me, I fell for Newcastle, I had great opportunities to leave during dark days, but that was too late I was already in love, I wanted, I needed, I had to stay for helping my team to save the club from Premier League relegation because it meant the world for me.
"Of course during those dark days some may have had their doubts, but I always believed in this club and look where we are today, it's just so beautiful. I'm leaving Newcastle as it should always have been and that's my greatest trophy because I was a part of it.
"A deep thank you to ALL of you, my teammates, the loyal fans, the staff, the Club, etc. I've seen all your messages, thank you and even though this chapter has ended, the story never stops.
"Newcastle is home now, I'll continue to be your biggest supporter, I'll watch every game and as long as the club keeps winning I'll be the happiest man in the world... and for sure you will see me again.
"I love you, I love you sincerely, I love you like a Geordie brother. Once a Geordie, always a Geordie."
Al-Ahli will compete in the Saudi Pro League next season following their promotion from the First Division, and they have already brought in the likes of Riyad Mahrez, Edouard Mendy and Roberto Firmino this summer.