The high court did not provide an explanation for why justices declined to hear Syed’s appeal.
Syed is serving a life sentence after a jury convicted him in 2000 of strangling his former girlfriend, Hae Min Lee, whose body was found in Baltimore.
Syed’s lawyer, Justin Brown, said on Monday he was “extremely disappointed” by the Supreme Court’s decision.
“Two courts have said he deserves a new trial, and then Maryland’s highest court reversed that. We think it’s appalling,” Brown told NPR. “We think this is a grave injustice. The way this has played out is sickening.”