Liverpool signed Southampton's Virgil van Dijk for a reported 75 million pounds ($100 million) in a deal that will make him the world's most expensive defender.

"Liverpool Football Club can confirm they have reached an agreement with Southampton for the transfer of Virgil van Dijk," a statement on the club's website said.

"The Reds have agreed a deal with the south coast club, and the player himself, that will see the 26-year-old defender move to Anfield when the transfer window re-opens on January 1, 2018," Liverpool said in a statement.

"The Netherlands international will be assigned squad number four, upon his arrival at Liverpool."

Southampton, meanwhile, also said they "agreed new world-record transfer fee" for a defender for Van Dijk.

The previous world record transfer for a defender was the 54 million pounds Manchester City spent on England right-back Kyle Walker in the close-season.

Van Dijk was expected to join the Merseyside club at the start of the season after he had handed in a transfer request to Southampton.

But the move fell through when Liverpool were forced to make a public apology after being accused of making an illegal approach for the player.