The zones affected by the designation are Edmonton, Calgary and central Alberta.
Shannon Evans, a spokeswoman for Alberta Health Services, says the declaration doesn't change how measles cases are managed.
But it does allow immunization of babies from six months to one year of age who normally would not be vaccinated until they were older.
The health agency is also advising that children four years of age and up receive their second dose of measles vaccine as soon as possible.
There have been almost two dozen confirmed cases of measles in Alberta this year.
Health officers determined that "in light of the continued recent cases ... (it) was a necessary step to protect public health," Evans said Tuesday in an email.
"With this determination made, an outbreak was declared, and the expanded immunization eligibility implemented, effective immediately."
Children, who have already received both doses of measles vaccine, do not require any additional vaccinations.