The former Oasis singer, 50, was in the stands to witness his club City conquer their rivals in the final.
The star was spotted carelessly rubbing salt in the wounds of a young United supporter as he celebrated his win at the all-important match.
Fans watching on BBC were shocked and in stitches at Gallagher’s behaviour, according to The Sun.
Liam Gallagher, 50, was spotted winding up a young Manchester United fan today during Manchester City’s FA Cup triumph at Wembley
The former Oasis singer was in the stands to witness his club City conquer their rivals in the final
One person said: ‘Liam Gallagher proper legend.’
Another commented: ‘Liam Gallagher s****housing a kid.’
Another person tweeted: ‘Lad loves it haha.’
It comes as a Manchester City supporter was seen punching YouTuber IShowSpeed during the final today.
The internet star was in attendance for Wembley’s clash and had been livestreaming himself watching the match from the stands.
All was going well until, midway through the first-half, a City supporter appeared to punch the streamer during his broadcast.
Liam’s appearance comes after the singer was seen arriving with his lookalike sons Lennon and Gene Gallagher at the Mulberry party at The Audley in London at the end of May.
Fans in stitches at Liam’s behaviour at the match took to Twitter to share their thoughts
The star was spotted carelessly rubbing salt in the wounds of a young United supporter as he celebrated his win at the all-important match
Liam’s brother Noel Gallagher (left) recently revealed there will be no Oasis reunion next year but there will be a new album from the band
His brother Noel Gallagher recently revealed there will be no Oasis reunion next year – after rumours of a possible reconciliation swirled around the internet.
However, it was reported that there will be a new album from the band.
The musician, 55, previously said his estranged brother Liam, 50, could ‘give me a ring’ and put their years of feuding behind them in order to take to the stage together once more.
Oasis split in 2009 after a backstage bust-up between arch-nemesis siblings Liam and Noel at their final concert in Paris, France.