A district court in southern Johor state found Tunku Nick Tazuddin Izwan Syah Tunku Qadir, who belongs to the state's royal family, and two other men guilty of a charge of voluntarily causing grievous hurt, defence lawyer Zamri Idrus said.
The three men were found guilty of assaulting boxer Farkhan Haron at 28-year-old Nick's residence in May last year.
Local media reports said Farkhan, the 2009 Laos Southeast Asian Games gold medallist, was attacked with wood, iron rods and a samurai sword, sustaining a broken jaw and causing him to miss last year's Olympic Games in London.
Zamri said the defence team would ask a higher court to release the trio from jail, while they prepare the appeal.
Allegations of wrong-doing by members of Malaysia's royalty and other elites surface occasionally, but it is rare for anyone to face prosecution.