Mertens signed for the Turkish side this week and will continue his career outside of Italy after nine years at the Stadio Diego Armando Maradona, or the Stadio San Paolo as it was known when he first arrived in 2013.
There were reports that Juve had tried to pick up the Belgium international as a free agent and those have now been confirmed by Galatasaray.
“Juventus offered him a contract worth €5m, I saw it with my own eyes,” said Galatasaray vice-president Erden Timur on the official club website.
“He didn’t want to end his career with the Bianconeri due to the rivalry with Napoli, so he refused their offer and decided to accept our proposal.”
A similar situation was also rumoured around Kalidou Koulibaly, who was wanted by Juventus when deciding not to extend his contract with Napoli, but opted to go abroad with Chelsea instead.