Martin had a contract with the Belgian Pro Continental team until the end of 2020, having started his professional career there in 2016, but they came to an agreement with Cofidis to release him early.
The 26-year-old won the under-23 Liège-Bastogne-Liège and a stage of the Tour de l'Avenir, both in 2015. This year, he finished 12th overall at the Tour de France, having placed 21st and 23rd in the two previous editions. He won the Giro della Toscana in 2017, the Circuit Sarthe in 2018, and a stage and second overall at the Giro di Sicilia this year.
"I'm very motivated by the idea of joining Cofidis. There's a lot to discover: a new set-up, new teammates, and a new start in my career. I have only ridden for one team as a professional, and I'm delighted to come to a French team.
"When Cedric told me about his interest in me, I was struck by his ambition and dynamism. I really want to take a step forward next year, with the aim of winning my first WorldTour race. I was very close this year at the Critérium du Dauphiné. Another wish is to ride two Grand Tours."
The signing of Martin represents another big move for Cofidis, that hopes to attain WorldTour status in 2020. They haven't had a top-10 finisher at the Tour de France since 2001, but in Martin they feel they have someone who can do that and more.
"Guillaume is an incredible rider who still has plenty of room for improvement, which could allow him to quickly start thinking about a podium on the roads of the Tour de France. I am convinced that we are going to help him take a step forward in his career," said Vasseur.
"I also spare a thought for all his supporters who wanted him to develop in a French team like ours. That's now done. I also have to thank the Wanty-Gobert team and their manager, Jean-Francois Bourlart, for giving him the chance to join our team from 2020."
Martin is Cofidis' third signing of the transfer window, following Viviani and his lead-out man Fabio Sabatini from Deceuninck-QuickStep. They are saying goodbye to Bert Van Lerberghe, who's going the other way, Hugo Hofstetter, who's off to Israel Cycling Academy, and Julien Simon, who's joining Total Direct Energie. They have handed contract extensions to Jesus Herrada, Natnael Berhane and Nicolas Edet.