Unless prosecutors decide to appeal the judge's order overturning Brendan Dassey's conviction, he could be set free in 90 days.
Dassey and his uncle Steven Avery were sentenced to life terms for the murder of a woman in 2005.
Their story was featured in the Netflix documentary series, prompting many to believe that they were improperly convicted.
The judge's order included strongly worded rebukes of the investigating police and of Leonard Kachinsky, the public defender appointed in 2006 to represent Dassey -- a 16-year-old minor at the time.
US Magistrate Judge William Duffin referred to Kachinsky's representation as misconduct that was "indefensible." He also said police investigators acted inappropriately toward Dassey.
"The investigators... assured Dassey 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.
Following release of the Netflix series, fans called for the two men to be freed and almost 130,000 people signed a White House petition asking for a presidential pardon.
"There is clear evidence that the Manitowoc County sheriff's department used improper methods," the petition said.
The White House explained that because the men were not convicted of federal crimes and "are both state prisoners," pardoning them was outside the scope of the president's power.
The documentary was prompted by the unusual story of Avery, who remains in prison serving a life sentence.
He was exonerated of a rape case through DNA evidence and freed from prison in 2003 after serving 18 years behind bars.
Two years later, while he was suing Manitowoc County in Wisconsin over wrongful imprisonment, he was arrested over the death of 25-year-old photographer Teresa Halbach.
Dassey was also charged with Halbach's murder and sentenced to life in prison.
