Oviedo have gradually lost their lustre amid deep financial troubles. They last played in the top flight in 2001 and are now down in the regional Segunda B (third tier) in fourth place, five points behind leaders Tenerife.
Many individual investors have responded to the club's appeal for cash in recent weeks. Real Madrid invested some £80,000 earlier this month to help the Asturian club keep afloat.
"We are at around 35 per cent (stake) which makes us, by far, the top holder," Slim's spokesman and chief aide Arturo Elias Ayub told Reuters on Saturday.
He said additional investments in the Spanish team were being received until midnight on Saturday.
The investment in Real Oviedo was made via Slim's privately held Inmobiliaria Carso.
The team "has a lot of qualities, has an impressive history, a strong following," Elias Ayub added. "More than a business investment, it was a personal investment (for Slim), a support to the team's fans."
Slim, who owns telecommunications, infrastructure, retail, mining and banking businesses, tapped the sports world in September after his mobile phone giant, America Movil, bought a 30 per cent stake in the Leon and Pachuca clubs in Mexico's first division for an undisclosed amount.
A Twitter campaign, fronted by Spain-based journalist Sid Lowe, has also been influential in inspiring supporters from all over the world to buy shares in Oviedo and help the club overcome their financial troubles.