The Baggies have now agreed a fee for Ajayi, believed to be in the region of £1.5million up front, although it could grow with add-ons.
Albion have beaten Barnsley to Ajayi's signature, and are set to complete a deal for the 25-year-old this week.
Earlier this week, Millers chief Paul Warne confirmed two Championship clubs bid for the ex-Arsenal defender, although the initial bids were rejected.
However, it's understood the Baggies have now had an offer accepted for Ajayi, who scored eight goals last season and can play defensive midfield or centre-back.
It comes after an £8m bid for Zohore was accepted by Cardiff, with the striker expected to be announced later today.
Albion's business is now beginning to pick up after a slow start to the transfer window.
Slaven Bilic is on the lookout for a defender after Craig Dawson’s departure earlier this window, joining Watford for around £6m, and Tosin Adarabioyo’s return to Manchester City after his loan spell at The Hawthorns.
Ajayi impressed for Rotherham in the Championship last campaign, although they ended up getting relegated, and won a supporters’ player of the year award.
Also capable of playing as a defensive midfielder, he featured in every match for the Millers last season.
The Nigeria international has also had spells at Cardiff, AFC Wimbledon and Crewe Alexandra.
Making defensive reinforcements is a high priority for Bilic, as is adding quality up front.
And it is thought Zohore is not the only striker Albion wish to bring in before the transfer deadline.
The aim is to get someone else in to make up for Dwight Gayle going back to Newcastle after his loan ended, Jay Rodriguez’s £10m return to Burnley and Salomon Rondon’s move to Chinese club Dalian Yifang.
Fine details are being gone over for Zohore. Should there be no hiccups, Albion will pay £8m if he reaches a set number of appearances, a set number of goals and helps the club win promotion back to the top flight.
The Danish striker only netted once in the Premier League last season, but scored 21 goals across two Championship seasons before that.
Zohore joined Cardiff in 2016 from KV Kortrijk, and was a key part of Neil Warnock’s promotion-winning side in the 2017/18 season – scoring nine goals and registering five assists.