Airports have been identified as a significant source of ultrafine particulate matter (UFP, particulate matter with diameter less than 0.1 μm), which may induce or aggravate pulmonary or cardio-respiratory health conditions, if prolonged exposure to high concentrations of UFP occur. Thus, assessing its impacts is vital to estimate UFP contribution to air quality degradation within the city and the degree of population exposure. However, there is lack of information regarding UFP concentrations in the vicinity of airports. This work aims to study the influence of air traffic and ground activities of Lisbon Airport (LA), in the surrounding urban area, focusing on the UFP concentrations. An UFP monitoring campaign was carried out in 2017 and 2018, for a 19 non-consecutive days period. The monitoring network was designed to include several sampling sites in the vicinity of LA and a set of sites further away of the LA, under the landing or take-off path. Based on the information collected, correlation analysis between air traffic activity and UFP concentrations was conducted. The results show the occurrence of high UFP concentrations in LA vicinity. Considering 10-min means, the particle counting increased 18–26-fold at locations near the airport, downwind, and 4-fold at locations up to 1 km distance to LA. Adverse orographic conditions leads to UFP punctual and average high concentrations. Results show that particle number increases with the number of flights and decreases with the distance to LA.