Alpena is a city in the U.S. state of Michigan and the county seat of Alpena County. It is considered to be part of Northern Michigan. The Thunder Bay National Marine Sanctuary is located offshore near the city. The population was 11,304 at the 2000 census. The population swells with a large number of visitors in the summer.Despite its small population, it is by far the largest city in the sparsely-populated Northeast Michigan (lower peninsula) area, serving as its commercial and cultural hub. It is considered to be one of the two anchor cities of Northern Michigan, along with Traverse City. The Alpena Regional Medical Center is a federally-designated rural regional medical referral center, and is the largest employer in the city.

According to the United States Census Bureau, the city has a total area of 23.5 km2 (9.1 sq mi). 8.4 square miles (21.7 km2) of it is land and 0.7 square miles of it is water. The city is on the shore of Lake Huron's Thunder Bay, with Alpena Township surrounding it on land.Access to natural resources and water transportation has been important in development of the community. Light stations and lighthouses surround Alpena. Several islands off the coast in Thunder Bay are part of the Michigan Islands National Wildlife Refuge, and Scarecrow Island is part of the Michigan Islands Wilderness Area. Charity, Little Charity, Scarecrow, and Thunder Bay Islands form the Lake Huron division of the Michigan Islands NWR. There are lighthouses on Charity (abandoned) and Thunder Bay Islands.