The Premier League club said it had the option to make the transfer permanent.
The 21-year-old Fernandes has played two games for Portugal, having represented his country at every age group from under-15s.
The arrival of Fernandes gives Tottenham another option in midfield after France international Moussa Sissoko was ruled out until April after undergoing knee surgery.
Unsettled midfielder Christian Eriksen has also been linked with a move away.
Source: Fox Sports