The Brazil international has now entered his fifth full season as a Real Madrid player after the 14-time European champions forked out £40.5m to sign him from Flamengo in 2018.
Vinicius initially struggled to live up to his potential at the Bernabeu, but he enjoyed a major breakthrough season last term with 22 goals and 20 assists from 52 outings in all tournaments.
The 22-year-old came up with 17 goals and 13 assists in 35 La Liga outings to help Real Madrid win the title, as well as scoring the winning goal against Liverpool in the Champions League final.
Vinicius has also started the new season brightly with three goals and two assists from five matches in all competitions, and he is thought to be close to signing a new long-term contract at the club.
Under La Liga rules, Real Madrid can only possess five non-EU players in their squad, as well as having a maximum of three included in a matchday team.
Previously, the Brazilian trio of Vinicius, Eder Militao and Rodrygo would occupy those three spots, which limited Real Madrid's movements in the summer transfer window.
However, a non-EU spot has now freed up in the Real Madrid squad after Los Blancos announced that Vinicius had obtained duel-nationality by adopting Spanish citizenship.
"Real Madrid CF announces that our Brazilian player Vinicius Junior swore to the Spanish Constitution last Friday, September 2, so from that moment he also has Spanish nationality," a statement read.
As a result, Militao and Rodrygo are the only two non-EU players in Carlo Ancelotti's squad, as Federico Valverde - who represents Uruguay internationally - also holds Spanish citizenship.
Japan international Takefusa Kubo also completed a permanent move to Real Sociedad this summer, while Brazilian youngster Reinier has been sent out on loan to Girona.
The Spanish and European champions are now free to pursue targets from non-EU countries in the January transfer window after being strongly linked with a summer move for Gabriel Jesus.
A move for the Brazilian attacker ultimately fell through due to the non-EU rule, though, and he swapped Manchester City for Arsenal in a £45m deal instead.
Jesus has a made a strong start to life at Arsenal with three goals and three assists in six Premier League outings, and the Gunners sit one point clear of Manchester City at the top of the table.
Meanwhile, Real Madrid - who only signed Antonio Rudiger and Aurelien Tchouameni in the summer window - have won all four of their La Liga matches so far to open up a two-point gap to Barcelona at the summit.