I was wondering, is a Dominican High School Diploma valid for studying college in Italy? I am Dominican and I want to study college in Italy, but I don't know if the diploma is valid. Considerating that the have 5 a year high school and we here in America have 4

Ontario has five year high schools, gr. 9, l0, ll, l2 and l3. Are you sure US has only 4 ?

I don't live in U.S.A. I'm from Dom Rep!

Yes, in the US high school is years 9, 10, 11, and 12. You should be able to enter higher education in Italy with your completion of high school no matter where it is from.
If you have a college or university in mind, check their website, or contact them directly, and find out what they may specifically require.