Betis, who are managed by former Hammers boss Manuel Pellegrini, have been targeting the midfielder throughout the January transfer window and had an initial bid knocked back earlier this week.
With hours to go until this evening’s deadline, however, the clubs have agreed a deal that will see the Spaniard’s wish of returning to his homeland granted.
The LaLiga outfit now have until the Spanish transfer deadline at 10pm GMT this evening to complete the deal.
There was confusion when Fornals was spotted at London Stadium ahead of tonight's clash with Bournemouth but it is understood he will undergo a medical in London before flying out on Friday morning.
Fornals was denied a move to Villarreal last summer, with David Moyes reluctant to weaken his squad, but he started just two of the Hammers’s first 19 Premier League games this term.
His contract was due to expire at the end of this season until West Ham activated a one-year extension late last year in a bid to protect his transfer value.
An injury crisis has seen Moyes turn to the 27-year-old to start games against Brighton and Sheffield United this month, but with Mohammed Kudus now back from the Africa Cup of Nations and Jarrod Bowen fit again, the Irons have decided to sanction his departure.
Fornals signed for the Hammers from Villarreal in 2019 and has gone on to make more than 200 appearances for the club, scoring 23 goals including a famous stoppage-time winner away to AZ Alkmaar in the semi-final of last season’s Europa Conference League.