Spain manager Luis Enrique (52) announced his 26-man squad for the tournament in Qatar, opting tp leave out experienced players like Ramos, midfielder Thiago Alcantara (31), defender Marcos Alonso (31) and goalkeeper David de Gea (32).
Ramos who joined PSG last year after 16 trophy-laden years at Real Madrid, wrote an emotional letter to fans after missing out on the chance to feature in his fifth World Cup.
"I devoted myself, body and soul, to recovering and getting back to feeling the way I always have... Fortunately, I can say that this season I feel like myself again and am back enjoying football," Ramos wrote on Instagram.
"The World Cup? Of course, it was one of those big dreams I wanted to realise. It would have been my fifth but unfortunately I'll have to watch it at home," Ramos said.
Ramos won the 2010 World Cup with Spain.
"It's tough, but the sun will still rise in the morning," he said. "I wouldn't change anything about myself. Not my mentality. Not my passion. Not my commitment, effort, and dedication."
Spain play Jordan in a friendly on Thursday ahead of their World Cup opener against Costa Rica.
