The 56-year-old, who saved Charlton from relegation from the Sky Bet Championship last season, agreed a contract in principle with the club’s chairman Karl Oyston last week.
Riga has now signed a one-year rolling contract to succeed former caretaker player-manager Barry Ferguson at Bloomfield Road.
He said: "I had a good discussion with the chairman and we found an agreement. I'm ready for the big challenge.
"It's not an easy challenge. I'm sure that with the support of the chairman and certainly the support of the fans - who really must be our twelfth man - I'm sure we can succeed.
"First of all, I will work hard to build a consistent team. They know the way I like to play but first of all we have to look for efficiency. The only promise I can make - which I've done everywhere - is to give all I can give to the club."
Riga, a former technical director at AC Milan, took over from sacked Charlton boss Chris Powell in March, charged with keeping them in the second tier despite sitting bottom of the division.
He won seven of his 16 games in charge and took them to an 18th-place finish, seven points clear of the drop zone, before leaving the club at the end of his temporary contract.
Riga has had two spells in charge of Standard Liege, who share the same owner as Charlton.
In his second stint during the 2011-12 season, Riga guided them to the last 16 of the Europa League before finishing fourth in the Belgian top flight.
Apart from his spells with Liege, he has also worked as a scout and analyst for the Belgian national team, spent time with the Aspire Academy in Qatar, and was invited by Jose Mourinho to spend a week training the Real Madrid squad in 2011.
Blackpool finished 20th in the Championship last season, after sacking Paul Ince in January, just two points above the relegation places.