Ranger, the former Newcastle and Blackpool striker, joined the Essex outfit in 2016 and scored 10 goals in 49 appearances.
“Following a meeting yesterday with Nile Ranger, the club has today terminated the player’s contract in accordance with the terms of the agreement,” a statement from Southend read.
“This decision was taken following reoccurring disciplinary issues, including timekeeping, which undermine both the unity of the group and the management.
“In many ways it is regrettable having worked closely with Nile over the past 18 months. However, the interests of the club are paramount and the board will never allow those objectives to be undermined.”
Ranger, 26, was released from prison in August after serving 10 weeks of an eight-month sentence for online bank fraud.
He scored two goals in 21 matches for the Shrimpers this season. His last appearance was on New Year’s Day as a second-half substitute in Southend’s 2-0 defeat at AFC Wimbledon.
England manager Gareth Southgate has claimed that the country needs to boot racism out of its own football before criticising World Cup hosts Russia.
Newcastle have been linked with a shock move for former Sunderland goalkeeper Simon Mignolet during the summer.