Janistyn made his first Austrian team at the 2008 Summer Olympics in Beijing, swimming in both long-distance freestyle and freestyle relay team events. He swam on the second leg of the men's 4 × 200 m freestyle relay, posting a split time of 1:50.48. Janistyn and his teammates Dominik Koll, Markus Rogan, and David Brandl finished heat two in fifth place and ninth overall, with a total time of 7:11.45. Three days later, Janistyn won the first heat of his only individual event, the 1500 m freestyle, by sixteen seconds ahead of Bulgarian swimmer and three-time Olympian Petar Stoychev, with a new Austrian record time of 15:12.46. Janistyn, however, failed to advance into the final, as he placed twenty-first out of 37 swimmers in the evening's preliminaries.