The former Tottenham and Fulham player was shown the red card for the first time in his career in bizarre circumstances
The Sounders were already down to 9 men in extra time after an earlier red card and injury that could not be substituted, when Micheal Azira was also dismissed for the home side for an elbow on a Portland player.
Infuriated by the decision, Dempsey marched up to referee Daniel Radford, seizing his notebook and throwing it away, before grabbing it again and ripping it up.
Radford sent the US star from the field, with the ex Fulham and Spurs man having to be restrained from confronting the referee again.
The MLS authorities had given Dempsey a three-match league ban and fine for referee abuse last week, but he also has a lengthier suspension and another fine from the Open Cup organisers that will last a minimum of six matches.
Dempsey's behaviour in the match was unlike the 32-year-old, with the red card he received the first he had received in his entire professional career.
Nevertheless, it is a ban that could mean he never plays in the US Open Cup ever again, with the forward either set to be absent for two years or six games, whatever is longest.
If Seattle go out in the first round in the next six seasons, he won't be able to play until 2022.
Despite the incident, Dempsey is still set to feature for the US in the Gold Cup in July.