BACKGROUND: Cable cars are part of the transport system in several cities in Latin America, but no evaluations of their effects on physical activity are available. TransMiCable is the first cable car in Bogotá, Colombia, and the wider intervention includes renovated parks and playgrounds. We assessed the effects of TransMiCable and the wider intervention on physical activity. METHODS: The Urban Transformations and Health natural experiment was a prospective quasi-experimental study conducted from Feb 1, 2018, to Dec 18, 2018 (baseline, pre-intervention) and from July 2, 2019, to March 15, 2020 (post-intervention follow-up) in the TransMiCable intervention area (Ciudad Bolívar settlement) and a control area without TransMiCable (San Cristóbal settlement). A multistage strategy was used to sample households in each area, with one adult (aged ≥18 years) per household invited to participate. Eligible participants had lived in the intervention or control areas for at least 2 years and were not planning to move within the next 2 years. Physical activity was assessed among participants in the intervention and control areas before and after the inauguration of TransMiCable in Ciudad Bolívar with the International Physical Activity Questionnaire (long form) and with wearable accelerometers. Complete cases (those with baseline and follow-up data) were included in analyses. Respondents were classed as being physically active if they met 2020 WHO guidelines (≥150 min per week of moderate activity, ≥75 min per week of vigorous activity, or equivalent combinations); and accelerometery data were classified with the Freedson cut-points for adults. Data were also gathered in zonal parks (area ≥10 000 m2) and neighbourhood parks (area <10 000 m2) in the intervention and control areas by direct observation with the System for Observing Play and Recreation in Communities, to assess levels of physical activity before and after the TransMiCable intervention. Multilevel regression models were used to assess changes in physical activity associated with the TransMiCable intervention. FINDINGS: Physical activity questionnaires were completed by 2052 adult participants (1289 [62·8%] women and 763 [37·2%] men; mean age 43·5 years [SD 17·7]) before the inauguration of TransMiCable. After the inauguration, the follow-up (final) questionnaire sample comprised 825 adults in the intervention group and 854 in the control group, including 357 adults in the intervention group and 334 in the control group with valid accelerometery data. 334 (40·5%) of 825 participants in the intervention group reported levels of physical activity that met the 2020 WHO guidelines during walking for transport before the intervention, and 426 (51·6%) afterwards (change 11·1 percentage points [95% CI 6·4 to 15·9]). A similar change was observed in the control group (change 8·0 percentage points [3·4 to 12·5]; adjusted odds ratio [OR] for the time-by-group interaction, intervention vs control group: 1·1 [95% CI 0·8 to 1·5], p=0·38). Time spent doing moderate-to-vigorous physical activity, measured with accelerometers, did not change in the intervention group after the inauguration of TransMiCable (change -0·8 min per day [-4·6 to 3·0]) and did not change compared with the control group (adjusted ß for the time-by-group interaction: 1·4 min per day [95% CI -2·0 to 4·9], p=0·41). Moderate-to-vigorous physical activity was 52·1 min per day (SD 24·7) before and 59·4 min per day (35·2) after the inauguration of TransMiCable in new regular users who reported using TransMiCable during mandatory trips for work or education (n=32; change 7·3 min per day [-22·5 to 7·9]). After the intervention, an increase in the proportion of male individuals engaging in moderate or vigorous physical activity was observed in a renovated zonal park (adjusted OR for the time-by-group interaction, intervention vs control park: 2·7 [1·1 to 6·8], p=0·033). Female users of a renovated neighbourhood park were less likely to become engaged in moderate or vigorous physical activity than female users of the control area neighbourhood park (adjusted OR for the time-by-group interaction: 0·4 [0·1 to 0·6], p=0·019). INTERPRETATION: It is encouraging that walking for transport remained high in the TransMiCable intervention area when the use of private motorised transport had increased elsewhere in Bogotá. In low-income urban areas, where transport-related walking is a necessity, transport interventions should be focused on efforts to maintain participation in active travel while improving conditions under which it occurs. FUNDING: Wellcome Trust (as part of the Urban Health in Latin America project); Bogotá Urban Planning Department; Ministry of Science, Technology, and Innovation of Colombia; Universidad de Los Andes; Fundación Santa Fe de Bogotá; and Universidad del Norte. TRANSLATION: For the Spanish translation of the abstract see Supplementary Materials section.