The forward arrived in Seville on Thursday to complete his medical and sign his new contract with the Verdiblancos, in a deal worth an initial €16m plus €4m in variables from AZ Alkmaar.
His arrival completes Betis's forward line and increases competition for places ahead of a demanding and exciting season for the Andalusian club, who will return to the Champions League for the first time in 21 years.
"Real Betis Balompié and AZ Alkmaar have reached an agreement for the transfer of the player Troy Parrott," the club said in a statement. "The Irish international forward becomes the fourth Verdiblanco signing of the summer and signs until 2031."
Born in Dublin on 4 February 2002, Parrott began his career at Belvedere FC in Ireland before joining Tottenham's academy in 2017. He went out on a series of loans in England, at Millwall, Ipswich, MK Dons and Preston, before making the move to Dutch football, where he played for Excelsior and, most recently, AZ Alkmaar, the club he arrives from.
His new signing follows a standout campaign in the Netherlands, contributing 31 goals and 12 assists last season, during which he won the KNVB Cup and Dutch Super Cup with AZ.
Parrott is also a regular fixture for the Republic of Ireland national team, with 37 caps and 11 goals to his name, including a hat-trick against Hungary earlier this year that helped send Ireland into World Cup qualification play-off contention.
Manuel Pellegrini's side had long targeted the 24-year-old as an option to strengthen their attacking options, with an English move to West Ham United having broken down in recent days as the Hammers turned their attention elsewhere.