Governor Lolo Matalasi Moliga said the declaration covered the entire territory.
Lolo said effective immediately, all visitors entering American Samoa through Samoa or Tonga must provide proof of MMR immunisation, as a condition of entry.
All American Samoan residents travelling to affected countries would also be subject to Health Department screening.
The emergency declaration is valid for 30 days with the possibility of extension.
There are currently two suspected cases of measles in the territory.