More than 200 people were killed in Wednesday's twister outbreak and several counties have major destruction.
Thursday's declaration means federal aid will be available to those who seek it.
The counties named in the declaration are Cullman, DeKalb, Franklin, Jefferson, Lawrence, Marshall, Tuscaloosa and Walker.
Assistance can include grants for temporary housing and home repairs and low-cost loans to cover uninsured property losses. Federal funding also is available to state and eligible local governments.