The One Direction singer is a Rams fan and has previously engaged in banter with the Elland Road club's Twitter account.
The East Midlanders had lost 1-0 in the first leg at Pride Park and, prior to kickoff, no second-tier team that had lost the first game at home had gone on to reach the final.
However, a 4-2 win for Frank Lampard's side in a dramatic game at Elland Road saw history being made as they triumphed 4-3 on aggregate and booked a date at Wembley.
Marcelo Bielsa's side had taken the lead when Stuart Dallas tapped home after Liam Cooper's header had hit the post but Jack Marriott levelled for the Rams.
Mason Mount and Harry Wilson netted after the break - the latter a penalty - but Dallas drifted in and fire Leeds back level on aggregate, only for Marriott's second to settle the tie.
Horan, best known for being in boyband One Direction and also a solo artist in his own right, has previously enjoyed some friendly banter with the Leeds Twitter account.
On Wednesday night, he tweeted the following:
Horan was also involved in an altogether more good-natured exchange with Derby goalkeeper Kelle Roos:
Letsss go— kelle roos (@kellsroos) 15 May 2019
♥️— Niall Horan (@NiallOfficial) 15 May 2019
Derby will face Aston Villa, who overcame West Bromwich Albion, in an all-Midlands Championship playoff final at Wembley on Monday 27 May.
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