Ranger has been plagued by off-field issues throughout his career and was released by the Blues in 2018 because of "reoccurring disciplinary issues".
The 29-year-old had been playing for Barnet Sunday League side AC United.
Ranger scored 10 goals in 49 appearances in his first spell with the Shrimpers between 2016 and 2018.
"I know Nile will divide opinion but I can only speak as I find and his attitude has been first class," Southend boss Mark Molesley told BBC Essex.
"He had to earn the chance to train at our training ground, he had to earn the chance to train with the group and he'll have to earn the chance to play.
"There are still some big hurdles in front of him but if he keeps showing the same attitude I think he's got all the potential and tools to be a really exciting signing for us.
"We are placing a lot of trust in him but with that comes a lot of life experience. I think Nile's at a stage now where he needs football more than football needs him."