Is it possible to lose weight without changing your diet?

I am not overweight; my weight is normal, but I still want to lose some fat and tighten up my muscles. I've joined a gym recently but I still haven't changed my diet in any way. Basically, I eat everything(sweets, chips...) but not too often, though, and I do train pretty hard at the gym. Will I be able to lose some weight this way or I have to make some changes in my diet?
Is it possible to lose weight without changing your diet?
Post Opinion