The 33-year-old has been without a club since last summer after being let go by the Hornets following their promotion to the Premier League. Doyley had spent his entire career with the Vicarage Road side, making over 400 appearances for the club after coming through the youth set-up.
The Jamaican international joins the Millers along with former Republic of Ireland goalkeeper Paddy Kenny, who has also signed a deal with the club until the end of the season.
It reunites Kenny, 37, with manager Neil Warnock, who he had worked with at a number of clubs in the likes of Bury, Sheffield United, Queens Park Rangers and Leeds.
Warnock was installed as the manager of the South Yorkshire outfit earlier this month following the sacking of Neil Redfearn.
The Millers are five points adrift of safety in the Championship and face high-flying Burnley at Turf Moor this Saturday.
What do you make to the news Rotherham fans? Happy with the signings? Let us know in the comments section below!