The 24-year-old joined the Shrimpers on a free transfer last summer following a spell with Colchester United.
Coker had one year left to run on his previous deal at Roots Hall before agreeing to the extension.
"I've really enjoyed my time here so far and I believe this club is moving in the right direction," he told Southend's official website.
"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."
Coker has played in all 42 of Southend's games so far this season.