"This is right, this is justice," his lawyer Laura Nirider said, adding that Dassey's family were "grateful, in shock".
The ruling could mean that his uncle Steven Avery, 54, who is serving life without parole for the 2005 murder of photographer Teresa Halbach, stands a chance of winning his fight for a new trial, and may even walk free ... all as the cameras roll for a second series of the hit show, which will air at Christmas.
The plight of Dassey and Avery, who are now household names, has attracted high-profile supporters including actors Susan Sarandon, Ricky Gervais and Mia Farrow.
Yet there is also considerable evidence against Avery, and some commentators now wonder - should he be released - just how much the decision will have been prompted by the mesmeric power of television drama and mob sentiment.
Filmmakers Laura Ricciardi and Moira Demos were initially drawn to the story after the Innocence Project, a group that has helped overturn dozens of wrongful convictions, began to champion the case. The story is told through police interrogations, recorded prison phone calls, interviews with family and lawyers and courtroom footage.
There is no narrator.
Averywas charged with the 2005 murder of Halbach, a 25-year-old who was last seen heading to take pictures of a vehicle at his yard for a car magazine. While police were convinced they had the right man - Halbach's burned remains had been found in a fire pit on Avery's property, and her car keys were discovered inside his mobile home - many believed he had been framed.
Avery's blood was found inside Halbach's car, but then a vial of his blood from a previous case was found to have been tampered with. The box in which it had been stored had the taped lid removed and a tiny needle mark was found in the top of the vial, suggesting blood could have been removed via a syringe and planted at the murder scene.
Manitowoc police officers, who at the time were in the middle of being deposed in his wrongful imprisonment lawsuit, had been involved in gathering evidence in the murder case, and defence lawyers insisted they might have planted evidence to frame him.
It was, however, the "confession" by Avery's then 16-year-old nephew Dassey that proved the most damning. The youngster who was "deeply impressionable", confessed that he raped Halbach and saw his uncle shoot her dead. He also admitted helping dismember and burn the body.
Dassey immediately recanted his confession, but it was too late and he was convicted of first-degree homicide.
In his ruling, Milwaukee judge William Duffin ordered Dassey, now 26, to be freed within 90 days unless prosecutors decide to retry him. The judge said investigators made false promises to Dassey by assuring him "he had nothing to worry about".
Not everyone is a fan of the Netflix show, particularly those concerned by the ever-increasing reach of television and, in particular, the internet, where debate has been raging.
Many have complained that Making a Murderer is unfairly sympathetic to Avery and Dassey, and leaves out vital evidence which points to their guilt.
Ken Kratz, who originally prosecuted the two men, is writing his own book about the crime - saying he wants to give a voice to the largely "forgotten" victim.
Kratz, whose reputation was tarnished by a 2014 sexting scandal, complained: "Any time you include only the statements or pieces of evidence that support your particular conclusion, then that conclusion could be reached. It really presents misinformation."
He claimed that vital evidence, including the fact that Avery called Halbach's work and specifically asked for the young, female photographer on the day of her murder, was left out of the show.
Avery's lawyer Kathleen Zellner said she had been visiting her client in prison when the news that his nephew's conviction had been overturned came through. She said he was "thrilled".
"We know when an unbiased court reviews all of the new evidence we have, Steven will have his conviction overturned as well," Zellner said, claiming new developments in scientific testing since Avery's 2007 conviction would "almost certainly" clear him.
Whatever twists and turns the case takes next, one thing is for sure: the cameras will be rolling to satisfy the appetite of millions of true-crime fans.
