The 31-year old Scotland international is set to sign a two-and-a-half-year deal with Viola believed to be worth around £700,000.
Reports suggest that Miller will sign by the end of the week to secure a salary of £40,000 per week and the deal could grow to around £1million overall with add-ons and bonuses.
Fiorentina have beaten Birmingham City to the signature of the former Celtic and Wolves man who reportedly said he did not want to turn his back on the Scottish Premier League for the English Premiership.
"I didn't think it was the right thing for him to be involved because there's an awful lot going on around him,” he said.
"He's in negotiations with Birmingham and Fiorentina and we don't know what the outcome of those talks will be.
"We don't know if he's going to leave, but hopefully the situation can be clarified in the next couple of days."