Riga City Kurzeme District Court on Friday sentenced the man named as Maksims Koptelovs to six months behind bars, his layer Ilona Bulgakov said, describing the verdict as a "shame for Latvia."
The full judgement in the case will be made public only on March 11, until which time Koptelovs has the right of appeal.
The judge in the case agreed with a prosecution request to send the accused to prison for launching an online petition asking Russia to take over Latvia, rather than opting for a more minor punishment.
The seriousness of the accusation - essentially involving an invitation to overthrow the state - merited time behind bars in the judge's view.
According to LSM, Koptelovs insisted his online appeal to Moscow, made nearly two years ago, was no more than a joke - one the court patently failed to see.
The petition attracted some 6,500 signatures before it was removed from the website in question.