The 24-year-old was granted two weeks holiday to celebrate his heroics for Argentina in Qatar after taking part in a victory parade around Buenos Aires on 21 December.
The Premier League club published a video on Monday showing the star returning to the club wearing his World Cup winning medal.
Mac Allister was seen beaming as he held aloft a replica of the World Cup trophy surrounded by players and staff as tickertape flutters around the lobby.
He became the first Brighton player to win the coveted trophy.
It is believed Mac Allister was due to return from his home in Santa Rosa ahead of Brighton's match against Middlesbrough in the FA Cup on Saturday, but could now feature in their Premier League clash at Everton on Tuesday evening.
His performances in Qatar has reportedly attracted the attention of Arsenal, Liverpool and Chelsea, who could offer bids for the player in the January transfer window.
