It urged Assad to free him, "given that his conviction violated his right to freedom of expression, in addition to his age and deteriorating health."
A similar appeal was issued by several Syrian human rights groups in July after Maleh was sentenced by a military court on charges of "publishing false information that could weaken national morale."
The July 4 ruling drew condemnation from the West, with the United States, European Union, France and Germany issuing calls for his release.
Maleh, who has worked for Amnesty International since 1989 and was involved in founding a Syrian rights group, was arrested on October 14 and questioned in a military tribunal over articles he had written.
He was already held behind bars between 1980 and 1986 along with a large number of trade union and opposition figures for issuing a call for constitutional reforms.
"President Assad surely cannot justify keeping behind bars an 80-year-old man who is only in jail for speaking his mind," said Joe Stork, HRW's deputy Middle East director.
"President Assad should show compassion during this month of Ramadan and release Haytham al-Maleh," he said.
Muslims last week began observing the holy month of Ramadan, during which they must fast from dawn to dusk, pray and be charitable.
HRW also said in the statement that Maleh's son informed the rights group that "his father recently had a stomach infection caused by prison food and was also suffering from degenerative arthritis in his knees.
"The results of a recent blood test showed he had an inflammatory disease, his son said," the statement added.
Maleh was arrested two days after an opposition Syrian television, Barada TV, aired a telephone interview with the human rights activist "in which he criticised the Syrian authorities," HRW added.
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