Adnan Syed's story became the nail-biting subject of the gripping podcast Serial after he was sentenced to life behind bars for murdering his ex-girlfriend Hae Min Lee.

Now, 18 years later, he is getting a new trial.

An appeals court in the US state of Maryland granted the new trial overnight in relation to the 1999 murder.

Because of the popularity of Serial, which became the most downloaded podcast in history, the case received worldwide attention.

However, the Maryland Court of Special Appeals found that Syed, now in his late 30s, received ineffective counsel and ordered that his 2000 conviction on charges of murder, kidnapping and false imprisonment be vacated.

Syed had been sentenced to life in prison for the murder of 18-year-old Hae Min Lee, whose strangled body was found buried in a shallow grave in the woods of Baltimore, Maryland.

Syed and Hae were honour students and children from immigrant families who had concealed their relationship from their conservative parents.

Prosecutors laid out a clear case, saying Syed's conservative Muslim upbringing made him feel especially humiliated, but his supporters said authorities had failed to contact a witness who claimed she saw Syed at the time of the murder in a public library.

"I haven't seen a single case in which an attorney failed to contact an alibi witness in which deficiency was not found," his lawyer C. Justin Brown told the Maryland Court of Special Appeals last year.

"This is an alibi witness who is providing their phone number and their willingness to testify, and the reason that this is so important is that there is no more powerful defence than an alibi."

The case earned new attention when it was taken up by Serial, a weekly podcast in which a US journalist revisited the case and cast doubt on his guilt.

The podcast — a mix of investigative journalism, first-person narrative and dramatic storytelling — focused its first season entirely on Syed's story in 12 nail-biting episodes. They were downloaded more than 175 million times, a world record.