The woman, 56, who was not herself married during the five-year affair with her neighbour, was told she must serve two years in jail or pay a fine of Tw$730,000 ($24,300).
But court officials said she had faced the possibility of 298 years in jail after judges used confessions from the pair to estimate they had held a total of 894 trysts in various motel rooms.
Under Taiwanese law, each offence was worth up to four months in jail, but judges at the district court in central Changhua county decided to reduce the sentence.
"Since the offence was not a felony, the judges decided to mete out what they thought was the proper punishment," Yu Shih-ming, the court's spokesman, told AFP.
The man, 50, avoided legal punishment altogether after his wife, who had filed the complaint against the duo after learning of the affair, decided to forgive him and drop the lawsuit against him.
The case sparked new calls for adultery to be decriminalised.
"Taiwan is one of the few countries in Asia where adultery remains a criminal offence," Lin Mei-hsun, deputy executive of the non-profit Modern Women's Foundation, told AFP.
"In the Changhua case, why was the woman punishable while her former lover escaped a legal punishment? This was unfair."
"To some extent, adultery should be decriminalised as we feel that women should have the right to decide who they love and who they have a sexual relationship with."
Taiwan's judicial authorities have been reluctant to drop adultery as a criminal offence, citing public opinion.
According to the latest survey done by the justice ministry in May, 77.3 percent of respondents said "no" when asked if they favoured the campaign to decriminalise adultery.
In a similar survey in April, 82 percent of people said they opposed decriminalisation, the ministry said.
"Many married women fear that once the criminal offence is removed, they will be short of a critical measure for preventing their husbands from having extramarital affairs," Lin said.
"But what they don't realise is that the criminal offence is not likely to ensure men's loyalty to their families."
The widow is not in custody as she decides whether to appeal the court's ruling.