I am interested in purchasing a balisong [A.K.A butterfly knife] online and was wondering if it is illegal in Singapore.

I don't know but do know that shipping is also a concern. Many countries don't allow them in the postal system. So you have 3 questions: country bought from, your postal regulations coming in, and whether you can own one and (carry it in public). I would ask your local police station first, and then if legal, go to the country postal websites directly where you want to buy one.