The 24-year-old only joined the Shrimpers during the summer, but he has impressed manager Phil Brown, who offered the left-back extended terms until 2017.
Speaking about his fresh deal, Coker told the club's official website: "I've really enjoyed my time here so far and I believe this club is moving in the right direction.
"I wanted to be part of that. I've been really impressed with how the manager works and believe he can really help me to continue developing as a player."
Prior to Southend, Coker has played for Bury Town, Histon, Colchester United and Chelmsford City.