Villa sacked Gerrard last Thursday after the Midalnds club collected just nine points from 11 games this season. Aaron Danks was appointed caretaker boss and Villa beat Brentford 4-0 over the weekend in his first game in charge.
"Aston Villa is delighted to announce the appointment of Unai Emery as the club's new head coach," Villa said in a statement.
"Unai will take over from November 1st after his work permit formalities are completed."
Emery, who replaced Arsene Wenger at Arsenal in 2018, lasted only 18 months at the North London club before he was sacked, paying the price for failing to reverse a decline in the club's fortunes.
He returned to Spain and has been in charge of LaLiga side Villarreal since July 2020 after signing a three-year deal. Spanish media reported that Villa paid the release clause to get Emery's signature.
While success eluded Emery in England, the 50-year-old has a proven track record of getting Spanish teams into the Champions League, finishing third in LaLiga three times with Valencia.
But his claim to fame is an ability to win Europa League titles, lifting the trophy three times with Sevilla and once with Villarreal to hold the record for most titles won by a manager in that competition.
Under Emery, Villarreal beat his former side Arsenal in the Europa League semi-finals last year before overcoming Manchester United on penalties in the final.
Villa are 14th in the standings with 12 points and next travel to fourth-placed Newcastle United on Saturday.
EMERY FAREWELL
A Villarreal statement said Emery had "unilaterally terminated his contract" and would hold a farewell news conference on Tuesday.
"Emery has gone down in Villarreal history as the first coach to win a title with the club, the Europa League, as well as the historic feat of reaching the Champions League semi-finals last season.
"Villarreal would like to thank Unai Emery for the work he has done and wish him the best of luck in his career."
Villarreal said the club's director of football Miguel Angel Tena would take over on an interim basis.
