Varun Chopra has joined Essex from Warwickshire earlier than expected and will be able to feature in their final two games of 2016.
The 29-year-old batsman had agreed to rejoin the county on a three-year deal after six years with the Bears.
"It was a place I always wished to come back to at some stage during my career," Chopra told the club website.
"Hopefully by using my Division One experience I can help the boys over the line and back into the top flight."
Division Two leaders Essex need just six points from their remaining two Championship games to win promotion, after second-placed Kent's defeat by Northants on Thursday.
Meanwhile, 18-year-old pace bowler Aaron Beard has signed a one-year contract extension at Chelmsford keeping him at the club until 2018.