After being appointed to lead the Seasiders in October, Campbell was unable to save them from relegation to League Two.
Campbell said: "I have really enjoyed my time at Southend, and whilst it was a difficult time fighting relegation, I still enjoyed the experience.
"We still managed to put together positive results, whilst also giving debuts to a number of the club's talented young players. Southend have a great group of young players coming through from the academy, which was evident in our 3-1 home win against Bristol Rovers in our last game before lockdown.
"There are obviously frustrations with how the season ended abruptly, and with what points we feel could have been amassed.
"However, the club have an exciting future with the plans for the new stadium in place, but with the current global COVID situation having a profound effect on businesses, I am keen not to be a financial burden to the club.
"I also want to thank the fans for their support throughout my time at Southend and wish the club and all the support staff the very best for the future."