Urban density is a term used in urban planning and urban design to refer to the number of people who inhabit a given urbanized area. As such, it should be distinguished from other measures of population density. Urban density is considered an important factor in understanding how cities function. Research related to urban density is conducted in various areas, including economics, health, innovation, psychology and geography, as well as sustainability.

It is often argued that higher density cities are more sustainable than low density cities. Much urban planning theory, particularly in North America, the United Kingdom, Australia and New Zealand, has been developed with the aim of raising urban densities, such as New Urbanism, Transit-oriented development and smart growth. However, the link between urban density and sustainability aspects remains a contested area of planning theory. Jan Gehl, leading urban designer and expert in sustainable urbanism, argues that dispersed and low-density cities are unsustainable since they depend on the automobile. A minority, like Randy O'Toole of the Libertarian Cato Institute, respond that increased densities translate into more expensive real estate, more road congestion, and more localized air pollution. Others respond that traffic congestion is not a result of population density but of parking capacity.[2]​ At a broader level, there is evidence indicating a strong negative correlation between a city's total energy consumption and its overall urban density, that is, the lower the density, the more energy is consumed.[3]​.

Urban density is a very specific measure of the population of an urbanized area, excluding non-urban land uses. Non-urban uses include regional open space, agriculture, and bodies of water.

There are a variety of other ways to measure the density of urban areas: