The 21 year old Portuguese ace only arrived at London Stadium last summer, but has suffered back-to-back relegations with Saints and the Hammers.
Despite the poor performances of his teams, Fernandes had individually caught the eye of the likes of Real Madrid, Arsenal and Manchester United.
But it is Spurs who have secured his signature, with The Athletic detailing that they will pay £85m guaranteed to add him to Roberto De Zerbi's squad.
At that price, Saints are set to receive a massive windfall due to a sell-on percentage clause they inserted into the deal to sell him for £42m last year.
That clause is reportedly worth around 15 per cent of the profit made by the Irons, which is around £43m. This equates to Saints receiving £6.45m.
The funds will be key as owner Dragan Solak looks to balance the books while maintaining as much of his Championship play-off final squad as possible.
Fernandes is said to be set for a medical with Spurs before the move, which will make him a fourth Spurs signing already this window, is made official.