According to the verdict yesterday, published by online newspaper Alaan, a tweet written by Rashid Saleh al Anzi in October "stabbed the rights and powers of the Emir" Sheikh Sabah al Ahmad al Sabah.
Anzi, who has 5700 Twitter followers, was expected to appeal, the lawyer said.
Kuwait, a US ally and major oil producer, has been taking a firmer line on politically sensitive comments aired on the internet.
Last year, a man was sentenced to 10 years in prison after he was convicted of endangering state security by insulting the Prophet Mohammed and the Sunni Muslim rulers of Saudi Arabia and Bahrain on social media.
Two months later, authorities detained Sheikh Meshaal al Malik Al Sabah, a member of the ruling family, over remarks on Twitter in which he accused authorities of corruption and called for political reform, a rights activist said.
While public demonstrations are common, Kuwait has avoided Arab Spring-style mass unrest.
But tensions have intensified between the hand-picked government, in which ruling family members hold the top posts, and the elected parliament and opposition groups.