The ruling upholds a previous decision to vacate Syed's 2000 conviction for the murder of his former girlfriend Hae Min Lee and remands the case back to Maryland's circuit court, where he would be retried.
Chapman said she was in the library next to their high school when she saw Syed at the time Lee was said to have been murdered. Lee's body was found in 1999 in a Baltimore park, following her disappearance weeks earlier.
On Thursday, the Maryland Court of Special Appeals determined he had received ineffective assistance by his counsel during the trial and upheld his request for a new trial.
Prosecutors declined to comment or to say whether they will appeal the ruling to the state's highest court.
Now, Christopher Wylie names JD(U), says Cambridge Analytica did caste surveys
On Tuesday, Wylie had told the British Parliament that his company worked in India and that he "believes" Congress was a client. It said in 2011, SCL India undertook a state-wide research campaign to identify voter caste by household.
Syed, who was the focus of the popular 2014 podcast 'Serial, ' has spent the last 18 years behind bars.
In a dissenting opinion, Judge Kathryn Grill Graeff disagreed with the majority, saying Syed had failed to overcome the presumption that his trial lawyer's failure to contact McClain was based on reasonable trial strategy.
Serial became one of the most popular podcasts to have ever been released and the fastest in iTunes history to reach 5 million downloads and streams.
Syed's attorney C. Justin Brown tweeted about the successful appeal on Thursday. Host Sarah Koenig and her team uncovered inconsistencies in his case - including the fact that Asia McLean was an alibi witness.
Brown said he is pleased by the ruling and said he hopes the state won't appeal it.
According to the Baltimore Sun, the office of the Maryland Attorney General is "currently reviewing today's decision to determine next steps". "We think we're going to win a trial".