Pellegrini is delighted to be reunited with club legend Joaquin, a player he coached during a successful spell with Malaga, as well as several others.
"Joaquin's career speaks for itself," he stated.
"At 38 or 39 years old, he is capable of making a difference, scoring eight goals this season.
"I'm very happy to coach him again, we had a great season together at Malaga.
"He brings experience, personality, joy and he will be important for the dressing room.
"I have no doubt that he can still bring many things to Betis.