You'll probably spot Pietrasanta Cathedral from a distance before you actually find this grand religious building. Look around inside if it's open or just admire the facade from the street.
You're spoiled for sightseeing choice when spending time in Pietrasanta. Pietrasanta Cathedral is a good place to start here, but there are numerous other fine attractions to discover. Forte dei Marmi Beach, Forte dei Marmi Fortress, and Pontile di Lido di Camaiore are not far from the principal church and will fill in your day nicely.