The Reds announced the move on Tuesday:
Caulker was previously on loan at Southampton for the first half of the current Premier League campaign.
The 24-year-old Englishman will provide much needed cover in the centre of manager Jurgen Klopp's defence, which has recently been crippled by injuries to Martin Skrtel, Dejan Lovren and Mamadou Sakho.
He made only three top-flight appearances for Ronald Koeman’s side in his short-term spell at St. Mary’s—two of them as a substitute, per WhoScored.com.
He has also been capped by England, and he will be available for Liverpool’s vital league clash with Arsenal at Anfield on Wednesday, per Andy Hunter of the Guardian, citing a statement on the club's official website.
However, Bleacher Report's Sam Tighe is sceptical of the move, especially with reports suggesting the Reds had to pay Southampton to cancel their loan agreement:
One previous reported centre-back target for the Reds was Neven Subotic, but Borussia Dortmund boss Thomas Tuchel recently ruled out any such move for the Serbian, per the Liverpool Echo's James Pearce.
However, Inter Milan's Andrea Ranocchia did look like a signing Liverpool could make, per Bleacher Report's Dean Jones:
Caulker will have to prove his worth at Anfield, as he will be under intense scrutiny but, given how desperately the Reds need cover at the back, Klopp will undoubtedly be relived to have made an early signing in January.