The Spanish midfielder will reunite with ex-Hammers boss Manuel Pellegrini in LaLiga, with a medical sorted in London on Thursday night.
The £6.8million deal initially looked to have been scuppered late on Thursday along with Said Benrahma's proposed £15m switch to Lyon, but Fornals has now officially joined Betis and West Ham team-mate Benrahma is also on track to move to France after late intervention from world governing body FIFA.
"West Ham United can confirm that Pablo Fornals has joined Real Betis on a permanent transfer for an undisclosed fee," West Ham said in a statement on Friday afternoon.
"Everyone at West Ham United would like to thank Pablo for everything he has given to the Club during his time in Claret and Blue, and wish him and his family all the best for the future."
After joining from Villarreal in a £24m deal in summer 2019, Fornals played over 200 times for West Ham across all competitions and featured in last season's Europa Conference League triumph, scoring the semi-final second-leg winner at AZ Alkmaar.
The east Londoners did not sign replacements for either Fornals or Benrahma on deadline day, landing only England midfielder Kalvin Phillips on loan from Manchester City earlier in the window.