Fernandes, 21, arrives at White Hart Lane as Mourinho's first move in the market since succeeding Mauricio Pochetinno in November.
The Portugal youth international will provide a welcome boost for the experienced tactician, with the club's midfield options depleted due to an array of injuries.
1. Erling Haaland - RB Salzburg to Borussia Dortmund (€20M)
2. Dejan Kulusevski - Atalanta to Juventus (€35M)
3. Exequiel Palacios - River Plate to Bayer Leverkusen (€17M)
4. Julian Weigl - Borussia Dortmund to Benfica (€20M)
5. Takumi Minamino - RB Salzburg to Liverpool (€10M)
6. Munas Dabbur - Sevilla to Hoffenheim (€12M)
7. Joao Pedro - Fluminense to Watford (€4M)
8. Carles Alena - FC Barcelona to Real Betis (Loan)
9. Mario Mandzukic - Juventus to Al Duhail (Free)
10. Zlatan Ibrahmimovic - LA Galaxy to AC Milan (Free)
"We are delighted to announce that we have reached agreement for the 18-month loan of Gedson Fernandes from Benfica with an option to make the transfer permanent," the club said in a statement.
Fernandes, who's been unveiled with the number 30 shirt featured 59 times for Benfica, scoring three goals and registering seven assists since graduating form the club's academy in 2018.