Speculation has been rife surrounding the future of the versatile 25-year-old throughout the summer, with manager Paul Warne confirming that two recent offers have been turned down from different clubs - speaking after his side's friendly win at Bradford Park Avenue on Wednesday night.
Albion have maintained their interest and made a breakthrough on Friday after the Millers agreed a fresh offer to sign the Londoner.
The fee is reportedly an initial £1.5m, with add-ons taking the deal to £2.5m, with Ajayi signing a four-year deal with the Midlands outfit.
The move is the second significant summer outgoing for the Millers, who sold captain Will Vaulks to Cardiff City worth an initial £2.1m up front, with the fee rising to £3.5m.
West Brom have been in the market for a centre-half after selling Craig Dawson to Watford for £6 million - while loanee Tosin Adarabioyo has returned to Manchester City.
Barnsley saw an offer rejected for Ajayi earlier this summer before switching their attention to other central defensive options, while Leeds United were also linked, although the reports appeared to be wide of the mark.
On the incoming front, bringing in a new goalkeeper remains the Millers' priority, with Leicester's Daniel Iversen having been consistently linked throughout the summer.