Derek Owusu's That Reminds Me follows the life of a boy called K from foster care to his birth family in Tottenham.
It has won the Desmond Elliott Prize, which is given to the year's best debut novel in the UK and Ireland.
Author Preti Taneja, who chaired the judges, said they were "as shattered by the truths of the story as we were moved by the talent of its writer".
Stormzy launched #Merky Books in 2018 to showcase writers "from all different walks of life, especially those who may have never had the opportunity to get into the industry so early".
Owusu, 32, did have a foothold in the industry, working part-time for #Merky's parent company Penguin Random House, and came to the attention of Stormzy's manager after editing a collection of essays titled Safe: On Black British Men Reclaiming Space.
That Reminds Me is his first work of fiction and is described as a novel in verse. The overarching story of K's life mirrors the author's own, from being in foster care in Suffolk to moving in with his biological family in north London before being diagnosed with borderline personality disorder.
He told BBC News why he created K as "almost an alter ego", how the publishing industry can improve on diversity, what Stormzy thinks of his book - and how he's also got the backing of Idris Elba.