The South African star, who moved to Northampton in 2017, has become a big fans' favourite during the past three seasons.
He has scored 31 tries in just 76 appearances for the club and is the reigning players' and supporters' player of the year.
But the lure of the Top 14 means Reinach will be moving on to pastures new this summer as he starts with his new club, Montpellier, on July 1.
And with rugby not due to return in England until after that date due to the ongoing Covid-19 pandemic, Reinach will not be able to play again for Saints.
It means his final appearance, though he didn't know it at the time, came at Worcester Warriors in March as Saints sealed a 16-10 Gallagher Premiership success at Sixways.
And with Reinach clearly sad about not being able to say goodbye as he wanted to, he sent a message to everyone via Saints' social media channels.
He said: "Okay, this is definitely not the way I wanted to say goodbye.
"Sitting here looking back at the last few seasons, I realised I'm extremely grateful for every opportunity I've had to put on the Saints jersey.
"Even though I gave my all to the jersey, it actually gave so much more back.
"For my family and I, this chapter is over. But I can tell you that it has been nothing short of spectacular and that has everything to do with the people around us.
"To every player, staff member and especially the supporters - thank you.
"I actually heard about the supporters before I got here. I heard they were mad about rugby and I didn't quite get it, but then I remember standing on the pitch at a packed Franklin's Gardens with the fans singing 'oh when the Saints go marching in', and I thought to myself 'I get it'.
"You guys are the heart of the team.
"And now to the boys... I wish I could be there with you guys to lift that trophy at Twickenham, but I'll make a plan to be there and celebrate with you afterwards.
"For now, all I can say is cheers, boys, it's been one hell of a ride."