The 44-year-old, who made 84 appearances for the Seasiders during his playing career, had been out of work since being sacked by Wigan Athletic last November.
He takes over at the New York Stadium on a three-year deal with the Millers currently sat bottom of the Championship table.
Richardson joins a number of ex-Blackpool players and coaches who are currently managing in the top five divisions of English football- ranging from the Premier League to National League North.
Here’s where you can find the former representatives:
7. Paul Simpson- Carlisle United
Current Carlisle United manager Paul Simpson helped the Cumbrians to League Two promotion via the Play-offs last season. At the weekend, the former winger returned to Bloomfield Road. During his time with the Seasiders, he made 35 appearances between 2000 and 2002. Photo: Paul Harding
8. Michael Flynn- Swindon Town
Michael Flynn is currently at the helm of Swindon Town, following previous stints with Newport County and Walsall. During his playing career he had two spells with Blackpool. After initially spending time on loan at Bloomfield Road in 2004, he returned permanently for a season in 2007- making a total of 39 appearances for the club. Photo: Charlotte Tattersall
9. Neil Wood- Salford City
Salford City boss Neil Wood is another former Blackpool representative who is currently in management. The 40-year-old initially joined the club on loan in 2006 before quickly making the move permanent- making a total of seven appearances. Photo: Ben Roberts Photo
10. Chris Beech- AFC Fylde
Chris Beech came through the youth system at Blackpool as a youngster, before joining Hartlepool United in 1996. He is currently the director of football and current caretaker manager at AFC Fylde. Photo: Lewis Storey
11. Micky Mellon- Oldham Athletic
Former Blackpool midfielder Micky Mellon has recently taken over as Oldham Athletic manager. The 51-year-old was at Bloomfield Road between 1994 and 1997. Photo: Lewis Storey