Riga, who was appointed in June has won just one of 15 games in all competitions, with 11 defeats.
"We couldn't leave the decision any longer to give someone else the opportunity to help us work our way out of the position we are currently in," said chairman Karl Oyston.
First-team coach Noel Blake will take charge of training on Tuesday.
The sacking ends a tumultuous spell for Riga, who took over a Seasiders squad that had just six first-team players when he arrived in early pre-season.
Despite assembling a new-look squad, Riga was unable to establish a settled side and questioned his chairman's commitment to signing new players.
Rumours regarding Riga's future at Bloomfield surrounded the club in recent weeks, with recently appointed Birmingham boss Gary Rowett among those interviewed by Oyston during Riga's reign.
Coincidentally the man Rowett replaced, Lee Clark, has also been suggested as a possible appointment.
Formerly with Charlton, whose Championship status he assured last term, Riga's tenure at Blackpool was overshadowed by a shortage of available players.
Only eight professionals were under contract two weeks before their first league game at Nottingham Forest, and despite signing 12 players in 12 days, they only had nine men eligible on the morning of the game.
The Seasiders eventually managed to name four substitutes for the City Ground trip, which they lost 2-0.
Victory against Cardiff and a draw with promotion chasers Middlesbrough and Wolves were rare highlights in an otherwise difficult campaign, while Saturday's 3-0 defeat by Reading left the club eight points adrift in the relegation places.
As well as their poor league start, Blackpool were also dumped out of the League Cup by League Two side Shrewsbury.
The Lancashire club, who were in the Premier League four seasons ago, are now searching for a fifth manager since Ian Holloway left for Crystal Palace in November 2012.