Dassey, 26, was convicted as a teenager, along with his uncle Steven Avery, of murdering Teresa Halbach in 2005. Both were convicted in 2007, and Dassey was sentenced to life in prison.
The story of the arrest and conviction of both men, including Avery -- who was wrongfully convicted of rape years earlier -- captivated the nation after "Making a Murderer" was released last year.
Some believed Avery, 54, was targeted by officials after he sued for $36 million in 2005 in the wake of his rape conviction being overturned, an allegation police and prosecutors vehemently denied.
Dassey's conviction raised eyebrows for some because of how his videotaped confession was conducted. At the time of the police interrogation, Dassey was a high school sophomore in Manitowoc County, Wisconsin. He had a low I.Q. and was enrolled in several special education classes.
In the Netflix docu-series, he was depicted as a confused and intellectually challenged teen, who often turned to his mother to help him comprehend his interactions with police.
In court documents obtained by ABC News, a judge ruled that Dassey's confession was obtained by investigators giving the then 16-year-old "false promises." The documents stated that "investigators repeatedly claimed to already know what happened...and assured Dassey that he had nothing to worry about."
Court documents also showed that during the four-hour interrogation, investigators told Dassey leading statements such as: "Tell us, and what else did you do? Come on. Something with the head, Brendan?"
Along with "Dassey’s age, intellectual deficits, and the absence of a supportive adult," the confession was ruled "involuntary."
"Dassey’s borderline to below average intellectual ability likely made him more susceptible to coercive pressures than a peer of higher intellect" and the teen lacked having "the benefit of an adult present to look out for his interests," the document said.
The documents also said that the "use of leading questions and disclosure of non-public facts makes it difficult to evaluate whether Dassey really knew the facts or was simply agreeing with the investigators." The ruling added that investigators only concluded their line of questioning after they had "indicated to Dassey that he finally gave the answer they were looking for."
According to Dassey's attorney Laura H. Nirider, the state has the option to appeal or retry the case within 90 days.
Still, "we are over the moon," she said. "[Dassey and his family are] grateful, in shock, trying to process this."
Dassey lived at the Avery family auto salvage yard next door to his uncle, who had been released from prison in 2003 after serving 18 years for a rape he didn’t commit.
Dassey’s confession was used as the main piece of evidence by prosecutors during the trial.
On the stand, Dassey insisted that he made up his confession to police, but he was still convicted of first-degree intentional homicide, mutilation of a corpse and first-degree sexual assault. He was serving a life sentence with a chance for early release in 2048.
Halbach was a freelance photographer, who had come to the Avery property to take pictures of a car Avery was selling.
ABC News reached out to former prosecutor Ken Kratz, who handled Avery and Dassey's murder trials, requesting comment, but hasn't received word back yet.