The island is largely covered by low mountains, while the lowlands are mainly comprised of muskeg, a type of soil made up of plants in various states of decomposition. The town is the 12th most lucrative fisheries port in the United States by volume according to the National Marine Fisheries Service. In 2004, 103 million pounds of fish and shellfish passed through Petersburg. Petersburg receives service from two separate ferry services: the Inter-Island Ferry Authority and the Alaska Marine Highway. The Inter-Island Ferry Authority has dedicated Friday, Saturday and Sunday service (May-October) with the M/V Stikine on its Northern Coffman Cove-Wrangell-Petersburg-Wrangell-Coffman Cove route.

In the city the population was spread out with 29.8% under the age of 18, 6.2% from 18 to 24, 30.6% from 25 to 44, 24.6% from 45 to 64, and 8.8% who were 65 years of age or older. The median age was 36 years. For every 100 females there were 108.7 males. For every 100 females age 18 and over, there were 108.0 males. The median income for a household in the city was $49,028, and the median income for a family was $54,934. Males had a median income of $42,135 versus $28,792 for females. The per capita income for the city was $25,827. About 3.3% of families and 5.0% of the population were below the poverty line, including 3.5% of those under age 18 and 4.2% of those age 65 or over.