Ex-"Deadliest Catch" star Jake Harris is sitting behind bars -- and has been for more than 3 weeks -- after he was arrested for felony DUI and drug possession ... TMZ has learned.

The incident went down back on January 23 in Skagit County, Washington where cops say the 33-year-old wouldn't identify himself to park rangers, who tried pulling him over. Harris took off with his RV, forcing state troopers to chase him down and force him to stop ... according to docs obtained by TMZ.

Cops say the trooper noticed signs of drug use once they pulled over Harris -- pinpoint pupils, bloodshot eyes and pale face. They also say he could not stand still ... shaking the entire time and talking fast. Meanwhile, a K-9 alerted cops to drugs in the RV.

Cops say they found a half ounce of heroin and other paraphernalia associated with the selling of drugs. Cops say they also found a shotgun Harris claims he bought from someone for $150 and a "ball" of heroin. Turns out it was stolen.

He was arrested and charged with 4 felonies -- DUI, possession of a controlled substance, maintaining a vehicle or premises for drug trafficking and possession of a stolen firearm. They also hit him driving on a suspended license.

Harris was skipper of the Cornelia Marie before leaving "Deadliest Catch" in 2012, and has struggled with drugs.