The 26-year-old, who helped Essex lift the first-class County Championship in 2017, will be available for eight games in the Vitality Blast tournament including their opening match against Middlesex at Lord's on July 18.
Expressing delight over re-joining the team, Amir said that he thoroughly enjoyed his stint with the team in 2017 and is looking forward to be a part of the squad again.
"I am very excited to return to Chelmsford and re-join my Essex teammates. I thoroughly enjoyed my time here in 2017 and I'm looking forward to playing my role in the Club's success this season," Essex Cricket official website quoted Amir, as saying.
Essex coach Anthony McGrath termed Amir as "one of the most exciting bowling talents" and hoped that his addition into the team would make their bowling attack "most dangerous" in the Vitality Blast tournament.
"Mo is one of the most exciting bowling talents in the world and I'm ecstatic he's going to be back with us. He has the ability to bowl at a fast pace whilst swinging the ball both ways and showed a real desire to come back to Chelmsford and be an Eagle again," McGrath said.
"Everyone has seen how devastating he can be with the ball and the talent he has, so I think alongside our other Overseas player, Adam Zampa, we're going to have one of the most dangerous bowling attacks in the Vitality Blast this year,' he added.
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