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:
1. Which former Blackpool representatives are currently managing in England?
Leam Richardson is back in the Championship after being appointed Rotherham United manager. Photo: George Wood
2. Rob Edwards- Luton Town
Rob Edwards is currently the manager of Luton Town, having helped the Hatters to the Premier League last season. The 40-year-old has also spent time with the likes of Forest Green Rovers and Watford since hanging up his boots. As a player, he made 67 appearances for Blackpool between 2008 and 2011- after joining the club from Wolves. Photo: Ian Walton
3. Leam Richardson- Rotherham United
After initially joining Blackpool on loan from Bolton Wanderers in 2002, Leam Richardson made the move permanent- remaining at Bloomfield Road until 2005. As a coach, he has previously held top roles with Accrington Stanley and Wigan Athletic. Photo: Charlotte Tattersall
4. Ian Evatt- Bolton Wanderers
Ian Evatt is the current manager of Bolton Wanderers, and has been at the Toughsheet Community Stadium since 2020. During his playing career, he spent a lengthy period at Bloomfield Road- making 254 appearances for the Seasiders. Photo: John Powell
5. Michael Appleton- Charlton Athletic
Michael Appleton has been Blackpool manager on two occasions, with his most recent stint coming to an end last January. Earlier this season, he took over the top job at Charlton Athletic. Photo: Tony Marshall
6. Richie Wellens- Leyton Orient
Richie Wellens is the current manager of Leyton Orient- and has previously spent time with the likes of Swindon Town and Salford City as well. As a player, he made 171 appearances for Blackpool between 2000 and 2005. Photo: Catherine Ivill