The news comes from Spanish outlet AS, whose report explains that Isco will also now earn a salary of €6m each season. His contract also comes with an optional fifth year to 2022.
Perhaps most interesting is the enormous new release clause. The midfielder's previous clause was rumoured to be €150m and increasing it by such a significant amount falls in line with Los Blancos ensuring that their player's clauses are seen as deterrents to buyers, not incentives.
A source told AS: "There are no players in Madrid's first-team squad with a clause below €500m, and in Castilla [Real's 'B' team], there are now players with clauses of €300m."
Real are obviously keen not to fall into the same trap as Clásico rivals Barcelona, who lost superstar Neymar to Paris Saint-Germain after the French club triggered his €222m clause. Even Lionel Messi only has a €300m figure written into his contract, which is no longer seen as an impossible fee.
After previously struggling to establish himself, Isco emerged as one of Real's key players in the closing stages of last season as the club captured La Liga and Champions League honours.
He was one of the stars of the show as Real then beat Manchester United in the UEFA Super Cup this week to kick off the new season with an early trophy.