Free-roaming urban cats, Felis silvestris catus, include feral cats (no owners), semiferal or strays (socialized toward humans) and domestic cats (not confined to owner property). In urban environments, free-roaming cats typically live in groups that might be directly influenced by the availability of food resources (often clumped cat lovers’ hand-outs). Apart from data available at animal welfare Non-Governmental Organizations (NGO), little is known about the free-roaming urban cats in Portugal. This study thus aims to describe and estimate the number and the density of free-roaming cats, and their colonies, within the city of Porto and to understand their spatial distribution in this urban habitat. Three different areas were selected within the city limits and surveyed, during three different years, for the presence of free-roaming cats. We found that free-roaming cat density is widely distributed within each study area. However, only a small number of colonies, most occupying small areas, were identified, with their location associated with food resources and shelter availability. Potential public health problems associated with high numbers of free-roaming cats in urban environments have already been recognized in other countries, but not in Portugal as far as we know. Awareness for the high density of cats in Portugal is important and should be considered a problem, which demands the promotion of free-roaming cat management programs (removal and/or neutering programs). Therefore, efforts for monitoring and understanding cat’s spatial ecology trends are fundamental.