The 26-year-old striker will replace Shane Long at St Mary’s after the Irishman’s move to AFC Bournemouth.
Saints head coach Ralph Hasenhuttl told the club’s website: “I am very pleased to welcome Takumi to our squad for the rest of this season.
“He will help provide another good attacking option, and is important the right profile of player for us.”
Minamino has scored four goals in 31 appearances for Liverpool.
— Southampton FC (@SouthamptonFC) February 2, 2021