The 41-year-old, who is the first Briton involved in the Ukrainian conflict to have faced charges, travelled out in 2015 to join pro-Russian rebels.
He was jailed for five years and four months at Manchester Crown Court.
Stimson, who went to the conflict zone in the Donbas region, admitted intending to commit acts of terrorism and engaging in their preparation.
Stimson - who smiled only once in court when he saw his friends and family on the balcony - left his home in August 2015 and travelled to Moscow before illegally entering eastern Ukraine.
He then joined a frontline unit of the Russian-backed militia fighting against Ukrainian government forces. He returned to the UK in November 2015 and was arrested upon his arrival at Manchester Airport. Among the items seized from him was his military clothing.
Stimson posted a photograph of himself on Facebook holding an AK-47 and summed up Donbas as "vodka, women and guns".
But Manchester Crown Court heard that he did not actually engage in any fighting during his four months in eastern Ukraine and had intended to perform humanitarian work by driving ambulances.
The judge agreed that Stimson had joined pro-Russian militia not to fight but to change the life he had in the UK.
In November 2015, he returned to the UK where he was arrested on arrival at Manchester Airport.
He was charged in September 2016 and remanded in custody.