PM_2.5 measurements using a network of lost-cost sensors were conducted during 2017–2019 in the greater area of Patras, Greece. The average PM_2.5 concentration in all sites during the study period was 9.4 μg m⁻³, varying from 6.2 μg m⁻³ in the background areas to 12.8 μg m⁻³ at the city center. The site with the peak PM_2.5 levels was not located in an area with high traffic density but rather in a square with pedestrian-only zones and a high restaurant density. The highest PM_2.5 concentrations were observed during the colder period (November–March) due to high emissions from residential wood burning for heating purposes. The measurements of the sensors were used to estimate the importance of regional and local PM_2.5 sources. During the warm period, regional transport dominated, contributing approximately 80–85% of the PM_2.5 in the city center; however, during the colder period, the local sources were responsible for approximately half the PM_2.5. The network operated reliably during this multiyear study. Such measurements provide, at a very low cost, valuable insights not only about the temporal and spatial variability of PM_2.5 in a city but also about its sources, including the role of regional transport.