Jatupat Boonpattararaksa pleaded guilty to charges of defaming the monarchy.
He was arrested for sharing the profile, published by BBC Thai, two days after the new king ascended the throne in December 2016.
Thailand has very strict lese-majeste laws, which ban any criticism of the country's monarchy.
Mr Jatupat, an opponent of the military-backed government who has taken part in numerous protests, is the only person to be prosecuted over the article.
He was one of more than 2,600 people to share the online profile of King Vajiralongkorn, 64.
In December, Mr Jatupat was charged with defaming the monarchy and detained in north-eastern Thailand. The court in Khon Kaen later denied at least 10 requests for his release on bail.