According to the Human Rights Watcher for Lao Religious Freedom (HRWLRF) group, Tiang Kwentianthong was imprisoned with four others last year, when he prayed for an elderly woman to be healed.
Authorities arrested him and subsequently found him guilty of conducting medical treatment without authorisation.
The married father-of six suffered from severe diabetes and died in hospital shortly after his release in September, after officials denied his request for treatment in jail.
The anti-persecution charity Christian Solidarity Worldwide told Premier: "We are deeply saddened by the news of Mr Tiang's death.
"It should be noted that this case has been riddled with inconsistencies and delays: since Mr Tiang and the other four Christians were first accused in June 2014, they have been arrested, accused of murder, cleared, accused under new charges, and convicted, pending appeal.
"For much of this period they have been kept in detention, away from their families.
"After being cleared of the original charges by the Savannakhet Office of Prosecutors in August 2014, Mr Tiang should have been released. Instead he was kept in prison, where his condition deteriorated.
"We urge the authorities to launch a full and impartial investigation into how this case has been handled and the circumstances surrounding Mr Tiang's death."
