The 56-year-old Belgian, who left Charlton when his contract expired at the end of the season, will sign the deal at the weekend.
Riga had only been in charge at The Valley since March but managed to save the club from relegation from the Championship with three wins in their final four games.
Blackpool have been without a manager since Barry Ferguson left his role as caretaker-boss, which he took on following Paul Ince's departure in January, at the end of the season.
Seasiders chairman Karl Oyston has been speaking to a number of candidates for the job.
Billy Davies, Brian McDermott, Brian Flynn, Paul Hart, Ricardo Moniz, Paul Cox and Jackie McNamara had all been linked with the position in recent weeks.