Madrid welcome Barcelona to the Santiago Bernabeu on Saturday, with the Catalan visitors 11 points clear of the Spanish and European champions.
Ramos' side do have a game in hand, though the captain understands the consequences of a Madrid defeat.
"It will be a great match," Ramos said at a sponsorship event on Tuesday. "There are no words to describe the Clasico.
"It is totally different, and the most important game in the world.
"We are obliged to win now more than ever due to the situation we find ourselves in, in order to remain in the fight for La Liga."
Madrid come into the match on decent form, having picked up the Club World Cup title thanks to a Cristian Ronaldo winner against Gremio in the final, and Ramos is hopeful the boost from that triumph can help lead his side to victory this weekend.
"We come into it in good form after winning the Club World Cup, which always suits the team on a psychological level," he added.
"We are eager to play and have been working hard this week, so hopefully we can get the three points that will allow us to stay close to those teams ahead of us."
Saturday's Clasico will be Madrid's final match of 2017, with the club having picked up five title in the calendar year but on the verge of letting Barcelona open up an near-insurmountable lead in the league.