Multiple reports suggest the One Direction star was formally offered the part, but chose not to take it.
Had he signed on, Harry would have joined Halle Bailey, who is already attached as Ariel.
The full cast is expected to be announced by director Rob Marshall at Disney's D23 Expo in California later this month (August 23-25).
Christopher Daniel Barnes, who voiced Prince Eric in the 1989 animated classic, was a big fan of the Styles casting, telling TMZ, "The guy's a rock star, actor, philanthropist... It's a pretty good start for a prince, right? I think he'll do a great job."