Trinity say the 26-year-old Scottish international failed a drugs test after last month’s Super League win at Widnes Vikings.
The news comes a day after Widnes revealed their former Castleford Tigers and England stand-off Rangi Chase tested positive for the same substance after the same game.
A Trinity statement - released at 11.15pm on Friday - said: “Wakefield Trinity received notification from UK Anti-Doping and the Rugby Football League today that Adam Walker tested positively for cocaine in a drugs test, following the recent fixture against Widnes Vikings.
“As a consequence, under Rugby Football League anti-doping regulations, he has been suspended from all training and playing activity.
“The Club fully respects this suspension and are currently reviewing this matter. There will be no further comment at this time.”
Bradford-born Walker began his career at Huddersfield Giants and then played for Hull KR before moving to St Helens after the 2016 season.
He failed to secure a place at Saints and joined Trinity on a two-and-a-half-year deal in May.
Walker was named in Trinity’s initial 19-man squad for Friday’s Super-8s game at Huddersfield, but did not play.