Adam Adli was charged last year after making several statements at a political meeting following national elections, including one in which he urged people "go down to the streets to seize back our power."
"Guilty," Adam tweeted from the court room. "Having a different opinion is apparently a crime now according to our dearest prosecutor."
Adam was freed on bail pending appeal to a higher court.
Sedition as defined by Malaysian law includes promoting hatred against the government.
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New York-based Human Rights Watch said Malaysia's ruling party, which has been in power in coalition with other parties since 1957, was "showing the kind of authoritarian tendencies one usually associates with single-party rule rather than democracy."
"More than anything, this conviction shows the incredible danger posed by the Sedition Act, which is so vague that it can be used by the government to criminalise any sort of speech it deems offensive," said its deputy Asia director Phil Robertson.
Prime Minister Najib Razak last year said the government planned to eventually abolish the Sedition Act, which was introduced in 1949 during British colonial rule, and replace it with new laws that would strike a better balance between allowing freedom of speech and ensuring public stability.
But he has appeared to have backtracked after his ruling coalition's poor performance in general elections in May last year. Najib's coalition won the polls but lost the popular vote for the first time to an opposition alliance headed by Anwar Ibrahim, a longtime thorn in the side of the ruling party.