Dassey and his uncle, Steven Avery, were convicted in separate trials of killing freelance photographer Teresa Halbach at Avery's home in Manitowoc County, Wisconsin.
Magistrate Judge William Duffin ruled that the guilty verdict returned by a trial jury in 2007 against Brendan Dassey was based on a coerced confession he gave as a 16-year-old youth with a learning disability.
Dassey and his uncle, Steven Avery, were convicted in separate trials of killing freelance photographer Teresa Halbach at Avery's home in Manitowoc County, Wisconsin. Halbach's charred remains were found in an incineration barrel and a burn pit on Avery's property, about 80 miles (130 km) north of Milwaukee.
The case was the subject of the 10-part Netflix-released documentary "Making a Murderer," which questioned the handling of the investigation and the motivation of Manitowoc County law enforcement officials, who sent Avery to prison in 1985 for a rape he did not commit.
Avery served 18 years in prison before DNA evidence exonerated him of the rape. He filed a $36 million federal lawsuit against the county, its former sheriff and district attorney in 2004.
A year later, Avery and his nephew were accused of killing Halbach. Dassey was convicted of first-degree intentional homicide, second-degree sexual assault and mutilation of a corpse. Avery was found guilty of first-degree intentional homicide and being a felon in possession of a gun.
The Emmy-nominated documentary suggests authorities planted evidence against both defendants, a claim rejected by the current sheriff.
In his ruling on Friday, Duffin wrote that misconduct by Dassey's own lawyer was "indefensible," including his permitting investigators to interrogate his client without being present.
The judge also faulted Dassey's interrogation by investigators, who assured the teenager "he would not be punished if he admitted participating in the offenses" and that "he had nothing to worry about."
"These repeated false promises, when considered in conjunction with all relevant factors, most especially Dassey's age, intellectual deficits, and the absence of a supportive adult, rendered Dassey's confession involuntary," Duffin said.
Dassey is set for release within 90 days unless prosecutors plan to retry him.
Avery has filed a notice of appeal in his own case. His attorneys were not immediately available for comment.