In the letter, posted on the website of his charitable foundation, Gorbachev said Ponsonov should be shown mercy because he was unaware he was committing a crime.
Ponsonov is the headmaster of a middle school in the remote Perm region, around 1,000 miles from Moscow in the Ural mountains.
He stands accused of violating Microsoft's intellectual property rules by using computers in his school that contained unlicensed copies of the firm's software.
Ponsonov claims the 16 computers delivered to the school in August 2005 were supplied by the regional administration and that he had no idea they were unlicensed.
Russia has faced much criticism for failing to protect intellectual property rights and has recently cracked down on piracy, attempting to clean up its image as it bids for membership of the World Trade Organisation.
That campaign has now widened in scope to target end-users of pirated materials and Ponsonov could face five years in prison and fines of up to $10,000.
The plight of Ponsonov has now attracted the support of Gorbachev, the former leader of the Soviet Union, who in his letter describes the headmaster as "a teacher, who has dedicated his life to the education of children and who receives a modest salary that does not bear comparison with the salaries of even regular staff in your company [Microsoft]".
In the letter, posted on the website www.gorby.ru, Gorbachev says Ponsonov "is threatened with detention in Siberian prison camps".
"We have great respect for the work of Microsoft's programmers ... and are in no way casting doubt on the principle of punishment for intellectual property violations. However, in this case we ask you to show mercy and withdraw your complaint against Alexander Ponosov," the letter read.
"This noble step will be enthusiastically received by all those in Russia who use Microsoft products."