At the turn of the XX-XXI centuries, post-socialist cities of Europe experienced an active transformation of their socio-demographic and economic structure. A striking feature of post-Soviet cities was the preservation of the disproportionate weight of industry in the economy against the background of a long absence of the real estate market. This phenomenon highlighted the need to solve the problems of socio-economic inequality within the city and restructuring its economy. This is especially true for Russian cities experiencing the shifts in the territorial structure of the population under the influence of transition to market economy, the third industrial revolution and the change of economic-geographical location. This study focuses on identifying trends in the social segmentation of the urban space of St. Petersburg as the second largest city in Russia and a socio-economic center of national importance. The social stratification of the city was studied at the grassroots administrative and territorial level based on the assessment of spatial distribution and the formation of territorial groups of the population with certain qualitative characteristics. The object of the study was 111 municipalities of St. Petersburg. The dynamics of their five most important indicators of demographic, social and economic development in 1989–2018 was analyzed: real estate tax on individuals per capita; the proportion of entrepreneurs; own incomes of municipalities per inhabitant; the proportion of people with an academic degree; cost of housing. Using the rank method, a social welfare rating was compiled. Information for the study was taken from the materials of the general urban planes of St. Petersburg in 1966, 1987 and 2005, the All-Russian population censuses of 2002 and 2010, the databases of the Federal State Statistics Service and the Federal Tax Service, and from the real estate «CIAN» company. The increasing social segregation by income was revealed. The existing differentiation of municipalities in terms of welfare is shown. The poorest are the municipalities of the southern part of the city (Kolpinsky, Nevsky, Krasnoselsky districts and Kronshtadt), while the most prosperous are the municipalities of Petrograd and Central districts, as well as certain territories of the municipal district of Moskovskaya Zastava, the villages of Komarovo, Repino and Solnechnoe.