The ruling comes as junta-run Thailand undergoes an unprecedented lese majeste crackdown, with convictions sky rocketing and record breaking jail sentences handed down as authorities broaden their interpretation of the law.
Chiranuch Premchaiporn, editor of the popular Prachatai news portal, was initially charged over 10 reader comments posted on the website in 2008.
She was convicted by Thailand's Criminal Court in 2012 which found that while she had not personally committed lese majeste, the 20 days she had taken to remove one of the comments fell foul of the law.
Today's final ruling upheld the 2012 conviction that carried an eight month suspended jail sentence and 20,000 baht (USD 550) fine.
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"I am disappointed with the verdict and I think the interpretation of the law has pushed a burden onto service operators," Chiranuch told AFP after the ruling.
Thailand has one of the world's harshest royal defamation laws.
Anyone convicted of insulting the revered but ailing 88-year-old King Bhumibol Adulyadej, or the queen, heir or regent can face up to 15 years in jail on each count.
Prosecutions have soared since the army, which styles itself as the champion of the monarchy, grabbed power in a coup last year.
Some of those who have fallen foul of the law have been given 20-30 year jail sentences, often for comments made on social media.
The law is also being increasingly broadly interpreted.
Earlier this month an auto-parts worker was charged with lese majeste for a Facebook post about the king's dog.
Academics have found themselves facing investigations for writing about past kings while Thai authorities recently confirmed that even "liking" a critical post on Facebook risks prosecution.
Critics say that situation often results in witch-hunts led by ultra-nationalists who comb through social media and monitor public events for possible breaches of the law.