Insiders claim the "Dunkirk" star is an early favourite for the role of Prince Eric in Disney's live-action revamp, which is being directed by Rob Marshall.
R&B singer Bailey is already attached as Ariel, the female lead, and Melissa McCarthy, Jacob Tremblay and Awkwafina are also in talks for parts, but producers now have their sights set on Harry as Eric, who becomes the title character's human love interest after she rescues him from drowning.
Prince Eric didn't sing in the 1989 animated movie, but the royal character was given a handful of songs when "The Little Mermaid" became a Broadway musical.
Meanwhile, Harry's casting news comes a day after he missed out on the role of Elvis Presley in filmmaker Baz Luhrmann's "Untitled Elvis Presley Biopic". Styles reportedly screen tested for the part but Luhrmann eventually went with Austin Butler.