Semi Ajayi leaves Rotherham United for West Brom
The Nigerian international has signed a four-year contract at the Hawthorns after spending two and a half years at AESSEAL New York Stadium.
The fee is believed to be an initial £1.5m, potentially rising by another £1m in add-ons.
Centre-half Ajayi, who can also play in midfield, made 108 appearances for Rotherham, scoring 13 goals.
The 25-year-old, signed on a free transfer from Cardiff City in 2017, could make an instant return to New York as the Championship Baggies are playing a pre-season friendly there against the League One Millers on Tuesday.
“He was in the last year of his contract, so it was good business for the club to allow him to move on.
“He came in as a very shy lad and was very raw. He did have the attributes I like in that he was athletic and he wanted to learn.
“He can play in a lot of positions and it is a massive blow that he has moved on. But unless you're the richest club in the country, your best players are always in danger of being picked off.
“Our job now is to find players who can develop and do the same thing that Semi has done for us over the last few years.”
Warne revealed on Wednesday evening that bids had been submitted by more than one second-tier side. Ajayi wasn't risked in a pre-season fixture against Bradford Park Avenue that night and was the only Millers substitute to remain on the bench.
A West Brom offer was finally accepted and the player underwent a medical yesterday.
Barnsley were another team willing to pay seven figures to land the former Arsenal youngster.
The Millers, who always acknowledged they would have to sell a player in the final year of his contract if the price was right, are now likely to up their interest in Blackpool central defender Curtis Tilt.
They had a bid accepted earlier in the summer before new owners at Bloomfield Road changed their minds and decided they wanted to keep the defender.
Armed with the Ajayi cash and the proceeds of the sale of Will Vaulks to Cardiff, Rotherham may now be prepared to test the Seasiders' resolve by increasing their offer.
Warne's men are in action in a friendly this afternoon against National League Chesterfield at the Proact Stadium (kick-off 3pm).