The 31-year-old leg-break bowler played 11 times in the competition for Northants last season, taking 12 wickets at an average of 24 runs.
Prasanna has appeared in 34 one-day and 14 Twenty20 internationals, including February’s series against Australia.
“The more match-winners on your teamsheet the better,” head coach David Ripley told BBC Radio Northampton.
“It’s his consistency with the ball, he’s been bowling the power-play against Australia and doing a good job of that.
“There’s some Sri Lankan cricket but at this stage it looks like he can pretty much do the whole competition, which is positive for continuity.” (Courtesy BBC)