After his contract with Ligue 1 club PSG ended in June, sources told Reuters that the 37-year-old snubbed a lucrative offer from Saudi Arabia's Al Ittihad and agreed a one-year deal with the Spanish side.
"It's a very special day, returning home is always a tremendous joy," said Ramos, who returns to Sevilla with the club bottom of the league.
"I am happy to return and try to contribute as soon as possible, which is the important thing."
The defender moved to PSG on a free transfer in 2021 after failing to reach an agreement to extend his 16-year stay at Real Madrid. He won two Ligue 1 titles during his two seasons with the French club.
At Real he won four Champions Leagues and five La Liga trophies, playing a total of 671 games and scoring 101 goals.
Spain's most capped international player also won the World Cup in 2010 and two European Championships, in 2008 and 2012, before announcing his international retirement in February.
"Sergio arrived at our youth academy at the age of seven and progressed through the youth ranks until he made his debut in the first reserve team at the age of 16 in the 2002/03 season," Sevilla said in a statement.
"Our academy's centre-back returns home almost two decades after leaving at the age of 19 for Real Madrid."