The Californian three-piece, made up of sisters Danielle, Alana and Este Haim, lead the 11th year of the rankings, which are compiled using tips from 213 UK-based bloggers, broadcasters and critics.
> Florence + the Machine, The Maccabees, Haim live at The O2, London - review
Radio 1 DJ Huw Stephens said: "Haim have an unapologetic passion for music that shines in their songs.
"They're brilliant musicians who have a real talent for tunes that merge their tales of growing up in the 21st Century with a classic songwriting ability. "
Haim said on Twitter: "Thank you, thank you, thank you!!! So excited/honoured to have won the BBC sound of 2013 poll!!! We kinda can't believe it.... xo"
The top five for the BBC Sound of 2013 was rounded out by AlunaGeorge, Angel Haze, Laura Mvula and CHVRCHES.
Other acts on the longlist included Palma Violets, Savages, A*M*E and Arlissa.
Last month, Haim were reported to be the second most-blogged-about act of 2012, just behind Sound of 2013 second-placed act AlunaGeorge.
Last year's BBC Sound of... list was topped by Michael Kiwanuka.
Watch the Haim 'Don't Save Me' music video below: