The Championship club have spent a number of weeks searching for a replacement for Tony Mowbray, who departed Ewood Park after failing to keep Rovers in a Championship playoff position during the closing weeks of the season.
Bringing in a new face for the dugout has developed into a drawn-out saga, Braga head coach Carlos Carvalhal and Everton first-team coach Duncan Ferguson both in the running at one stage.
However, Blackburn have instead decided to appoint Tomasson, who last enjoyed a hugely successful period at Swedish club Malmo.
As well as winning league titles during his two-year reign, the 45-year-old guided Malmo into the Champions League group stages, beating Rangers during the qualifying rounds.
Tomasson departed his role at Malmo at the end of last year, and the Dane says that he is now ready to take his opportunity to manage in England.
Speaking to the club's official website, Tomasson said: "I'm really proud and excited to be taking over as Head Coach of Blackburn Rovers - a club with a lot of tradition and also great ambition. We have a young team here, and also a great academy, and the owners have a clear vision, which is to develop players and become a sustainable Premier League club over time, so I'm really happy to be involved in this exciting new chapter for the club.
"Since leaving Malmo, I have been looking for the right club and I had a lot of options, but after speaking to Steve [Waggott] and Gregg [Broughton], I got a really good feeling about the club and I knew that I had found my next opportunity.
"I grew up watching English football, so I know all about the passion for football here and how special football fans are in England, so it's great to come to such a big club as Blackburn Rovers and to be involved with such passionate fans and in a really good league, which the Championship is. It will be a great challenge, but it's something I'm really looking forward to being a part of."
Blackburn say that Tomasson will be joined by assistant coach Remy Reijnierse and performance director Ben Rosen, with David Lowe, Damien Johnson and Ben Benson being part of the coaching setup.
As well as watching English football during his childhood, Tomasson is no stranger to the Premier League having spent a period representing Newcastle United during his playing career.
The Magpies paid £2.2m to sign the forward from Heerenveen in 1997, Tomasson being regarded as an important addition ahead of the club's Champions League campaign.
However, just four goals were scored from 34 appearances in all competitions, 26 of those outings coming from the starting lineup.
Tomasson, who is Denmark's joint-record highest goalscorer with 52 strikes, went on to win the Eredivise title with Feyenoord and Serie A and Champions league crowns with AC Milan.