The 26-year-old striker was jailed in May after admitting online banking fraud in which a woman lost more than £2,000.
Ranger penned a new contract at Roots Hall until 2020 in December and Southend have confirmed that he is available to return to training with Phil Brown's side immediately.
A statement published on the League One club's website read: "Nile Ranger was Friday morning released from Pentonville Prison after serving a little over 10 weeks.
"Nile was eligible for early release under the HDC (Home Detention Curfew) Scheme, due to the short duration of his sentence and his consistently excellent custodial behaviour.
"Nile may regard himself as fortunate but will remain under an evening home curfew for the next five to six weeks.
"However, this will enable him to resume training immediately and Nile Ranger is today undergoing a fitness test and assessment that will help steer his training regime over the next seven to 10 days before he rejoins the squad."
Ranger has previously spent time with Newcastle United, Barnsley, Sheffield Wednesday, Swindon Town and Blackpool.