I am an American citizen and own a

Yes, I’m afraid it is. Any income made from a property in Italy needs to be declared in an Italian tax return, and then generally also in your domestic return, where you should receive credit for the tax paid in Italy.Note that this doesn’t just apply to US citizens – it applies equally to residents from any other country, EU or not. The returns for 2014 income are due in by the end of September – let us know if you have any queries or need our assistance. 

Yes, Italian law says that the property earns you money in Italy and therefore its Italian income.   As a nonresident homeowner you could also have to fill in a tax return in Italy anyway - dependent on your catastal value and IMU payments.