Powell, 30, joins the Riders after spending the past four seasons with the Ottawa Redblacks.
The five-foot-eight, 230 pounder started 16 regular-season games last year, picking up 1,362 yards and six touchdowns on 251 carries.
He added 39 catches for 319 yards and two touchdowns.
Powell has 500 carries for 2,835 yards and 13 touchdowns in 35 career regular-season games.
The Texas native is a two-time East Division All-Star and won a Grey Cup with the Redblacks in 2016.
The Roughriders also signed international linebacker DyShawn Davis to a one-year deal.
Davis spent the last three seasons with the B.C Lions and finished fourth in the CFL with 22 special-teams tackles in 17 regular-season games last year.
The six-foot-two 230 pounder also had 13 defensive tackles, two fumble recoveries and one forced fumble.
© 2019 Global News, a division of Corus Entertainment Inc.