Reguilon starred on loan with Sevilla last season and the Europa League winners had hoped to re-sign him from Real Madrid for the 2020-21 campaign.
However, sporting director Monchi conceded last week his side would likely miss out on the 23-year-old, who is also a reported target for Manchester United.
And on Monday the LaLiga outfit confirmed the arrival of Argentina international Acuna on a four-year deal for a reported upfront fee of €10million.
Acuna spent three seasons with Sporting, making 134 appearances for the Portuguese giants in all competitions.
He becomes Sevilla's fifth signing of the window following deals for Ivan Rakitic, Oscar Rodriguez, Yassine Bounou and Suso, the latter two making their loan moves permanent.