It was the third straight year the Winterhawks had advanced to the final, but first time they've claimed the Ed Chynoweth Cup.
Two of Rattie's goals came while the Winterhawks were short-handed. Oliver Bjorkstrand and Taylor Leier also had goals for Portland.
Travis Ewanyk had the lone goal for Edmonton.
The Western Hockey League in November suspended Winterhawks coach and general manager Mike Johnston for the rest of the season, fined the team $200,000 and took away several draft picks Wednesday because of player-benefit violations over the past four seasons.
The Winterhawks advanced to the Memorial Cup later this month in Saskatoon, Saskatchewan.