The Saints scrum-half officially joins the French giants on a three-year deal on July 1, having enjoyed huge success in Northampton.
Reinach, who joined Saints from Super Rugby side the Sharks in the summer of 2017, scored 31 tries in 76 appearances for the black, green and gold.
He is the reigning players' and supporters' player of the year.
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But he will now be heading to pastures new, explaining that his continued growth as a player was a key reason for the move to the Top 14.
"I’m so stoked about this move and really happy to finally be able to talk about it," World Cup winner Reinach told SARugbymag.co.za.
"It’s been the worst-kept secret of the year!
"I’m at a stage in my career where I have to be aggressive about the growth I’m chasing.
"Ironically, it was my move to Northampton that showed me just how important choosing the right club at the right time is.
"So, even though my family and I are extremely happy here, I’m leaving Northampton with a rustige hart (peaceful heart) because one, it’s the right time – I gave absolutely everything I had to give, and I learned everything the Saints have to teach.
"And two, I believe I’m heading to the right place – Montpellier and the Top 14 offer the opportunities I require at this stage to grow as a player."
Montpellier aim to be perennial title challengers in the Top 14 and boast a wealth of talent.
And Reinach said: "I don’t think the Top 14 requires a different skill set to the Premiership. It’s not like when I came from the south to the north.
"Like I said previously, in my personal opinion, the Top 14 is currently the toughest competition at club level.
"The intensity of the competition is a big attraction for me.
"If you look at the names the Top 14 attracts, it’s impossible to not want to be a part of that.
"Stepping on to the field with those guys you will have no choice but to lift and improve every aspect of your game."