Ajayi has signed a four-year deal with the Championship side.
Ajayi had his best individual year with Rotherham United in the 2018/2019 season with eight goals in all competitions and the award as the club’s Player of the Season.
His performances could however not save Rotherham from relegation to League One but the 25-year-old will remain in the Championship with West Brom after signing a four-year deal with the Baggies.
Semi Ajayi signed a four-year deal with the Championship side (Twitter/West Brom)
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“I got a whisper towards the end of last season that Albion were watching me although I didn’t hear any more,” Ajayi told club website.
“I went away to play for Nigeria and thought nothing more of it but when I came back things began to start rumbling again.
“I loved the fact that they were so keen to sign me and I’ve come into the club and found really nice facilities and good people. The lads have been great welcoming me.
“I know I’m not the finished article and I feel I can improve. I am playing alongside some top defenders and I think I can learn a great deal from them.
“For me it doesn’t matter where you are in the league – top or bottom. I want to win every game and I prepare like that. But I want to play at the highest level and it’s exciting to be joining a club that wants to get back there.”
It is yet not known where Ajayi will fit in Slaven Bilic’s West Brom after playing both as a defender and midfielder for Rotherham United. He, however, played most games of the 2018/2019 season in midfield
“I want to play my part in helping the team get promoted because that is definitely the target.”
Ajayi becomes the seventh Nigerian to play for West Brom after Super Eagles great Kanu Nwankwo, Sam Sodje, Victor Anichebe, Osaze Odemwingie, Ifeanyi Udeze and Brown Ideye.
Born in London to Nigerian parents, Ajayi played his youth football at Charlton and Arsenal.
His professional career kicked off at Cardiff City and he has since played at AFC Wimbledon, Crewe Alexandra and Rotherham.
He has six caps for the Super Eagles of Nigeria but could not make the squad ahead of the 2019 Africa Cup of Nations (AFCON) in Egypt.