The city was co-founded by John C. Williams, formerly of Detroit, Michigan, who purchased the land in 1876, and by Peter Demens, who was instrumental in bringing the terminus of a railroad there in 1888. St. Petersburg was incorporated on February 29, 1892, when it had a population of only some 300 people.
It was named after Saint Petersburg, Russia, the birthplace of Peter Demens. A local legend says that John C. Williams and Peter Demens flipped a coin to see who would have the honor of naming the city. Peter Demens won and named the city after his birthplace, while John C. Williams named the first hotel after his birthplace, Detroit. The Detroit Hotel still exists downtown.
The city's first major industry was born in 1899 when Henry W. Hibbs, a native of Newport, N.C., established his wholesale fish business at the end of the railroad pier, which extended out to the shipping channel. Within a year, Hibbs Fish Company was shipping more than 1,000 pounds (454 kg) of fish each day.
Dredging of a deeper shipping channel from 1906 to 1908 opened St. Petersburg to larger shipping. Further dredging improved the port facilities through the 1910s. By then the city's population had quadrupled to 4,127.
In 1914, airplane service across Tampa Bay from St. Petersburg to Tampa and back was initiated, generally considered the first commercial airline. The company name was the "Airboat Line" and the pilot was Tony Jannus. The Tony Jannus award is presented annually for prestigious work in the airline industry. There is also a very popular local music/entertainment venue named after Tony Jannus called Jannus Landing on Central Avenue in Downtown. Air service was discontinued in 1924 with the opening of Gandy Bridge, the first across Tampa Bay, which allowed automobile traffic between the two cities.
The city population continued to multiply during the twentieth century through the 1970s as the town became a popular retirement destination for Americans from midwestern cities, reaching 238,647 in the 1980 census. By the 1980s, however, the population had levelled off, and has grown by only 10,000 since then, primarily as a result of being "built-out".
As of the censusGR2 of 2000, there were 248,232 people, 109,663 households, and 61,630 families residing in the city. The population density was 4,163.1 persons per square mile (1,607.3/km²). There were 124,618 housing units at an average density of 2,090.0 per square mile (806.9/km²).
There were 109,663 households out of which 24.0% had children under the age of 18 living with them, 38.3% were married couples living together, 13.8% had a female householder with no husband present, and 43.8% were non-families. 35.6% of all households were made up of individuals and 13.1% had someone living alone who was 65 years of age or older. The average household size was 2.20 and the average family size was 2.88.
In the city the population was spread out with 21.5% under the age of 18, 7.7% from 18 to 24, 30.2% from 25 to 44, 23.1% from 45 to 64, and 17.4% who were 65 years of age or older. The median age was 39 years. For every 100 females there were 91.2 males. For every 100 females age 18 and over, there were 87.7 males.
The median income for a household in the city was $34,597, and the median income for a family was $43,198. Males had a median income of $30,794 versus $25,860 for females. The per capita income for the city was $21,107. About 9.2% of families and 13.3% of the population were below the poverty line, including 19.1% of those under age 18 and 10.8% of those age 65 or over.