Brendan Dassey, a subject of the Netflix documentary series “Making a Murderer,” won a major court victory on Thursday, as a federal appeals court upheld an earlier ruling by a federal judge overturning his conviction. The ruling came 10 years after Mr. Dassey was convicted of sexually assaulting and killing a 25-year-old photographer.

In a 2-to-1 decision, a panel of the United States Court of Appeals for the Seventh Circuit said that it agreed with the ruling last August by William E. Duffin, a federal judge for the Eastern District of Wisconsin. The judge had found that Mr. Dassey was coerced into a confession in which he admitted that he assisted his uncle, Steven Avery, in sexually assaulting and killing the photographer, Teresa Halbach. Mr. Dassey later recanted the confession.

“In addition to failing to apply a totality of the circumstances analysis to the facts of this case, as required by the Supreme Court, the state court acted unreasonably when it determined that, given the totality of the circumstances, Dassey’s confession was voluntary,” Judge Ilana Rovner wrote for the majority in a 104-page opinion.

But the real-life drama continues for Mr. Dassey, 27. For now, he will remain in jail, pending the Wisconsin Department of Justice’s next move. It has indicated that it wants to appeal the panel’s decision.

“We are evaluating the 2-1 decision from the court,” Johnny Koremenos, the communications director for the Wisconsin Department of Justice, said in a statement on Thursday. “We anticipate seeking review by the entire 7th Circuit or the United States Supreme Court and hope that today’s erroneous decision will be reversed.”

He added, “We continue to send our condolences to the Halbach family as they have to suffer through another attempt by Mr. Dassey to re-litigate his guilty verdict and sentence.”

In the meantime, Mr. Dassey’s lawyers said they would again move to have Mr. Dassey freed pending further action in the case. He was originally sentenced to life in prison.

“We’re absolutely thrilled,” said Laura H. Nirider, a member of Mr. Dassey’s legal team, in an interview. “We’re over the moon for Brandon. We’re over the moon for his family. This was a victory for justice and the truth today.”

In addition to providing evidence that Mr. Dassey was coerced into a confession, “Making a Murderer,” a 10-part series by Laura Ricciardi and Moira Demos released in December 2015, suggested that Mr. Dassey was improperly questioned by police investigators in 2006, when he was 16, and that he was mentally unfit.

Last August, Judge Duffin overturned his guilty verdict, which Brad Schimel, the attorney general in Wisconsin, appealed the next month.

A lawyer for Mr. Avery, who is currently serving a life sentence, filed a motion this month, citing ineffective counsel as a reason for Mr. Avery to be freed or granted a new trial.