Mammal species such as shrews, mice, voles, red tree squirrels, mink, and river otters inhabit the park. Brown bears occupy the Indian River drainage and occasionally enter the park, often at the beginning of the salmon runs. Sitka blacktail deer also occasionally enter and forage in the park but are discouraged from doing so because of the development and human activity around the park. However, visitors observed the birth of a fawn on the morning of June 3, 1999 at the fort site.

Marine mammal species don't live within the park, however many species are observed in nearby waters.  Humpback whales, steller sea lions, sea otters, harbor seals, killer whales, gray whales, Pacific white-sided dolphins, harbor porpoises, Dall's porpoises, and northern elephant seals have been sighted near the park.

Threatened or Endangered Species
No endangered or threatened mammal species is known to inhabit the park. The endangered humpback whale and the threatened Steller sea lion are commonly sighted in marine waters around Sitka.