The 25-year-old, who has played 30 NRL matches for the Storm, has signed a two-year contract with the Cowboys.
Kostjasyn made his debut for Melbourne in round one of the 2010 season and is able to fill a number of roles, including hooker, back-row and in the halves.
His 12 matches for the Storm this season have included a stint as starting five-eighth.
Cowboys General Manager – Football Peter Parr said Kostjasyn’s versatility would be an important factor for North Queensland’s next two NRL campaigns.
“Rory has shown during his time with the Storm that he’s capable of doing a very good job in a number of positions,” Parr said.
“No doubt he will come here with a goal of nailing down a permanent position in our top 17.
“We expect he will make a big contribution to the Cowboys from day one.”
Kostjasyn is the second new face coming to North Queensland next season.