BACKGROUND Maternal overweight and obesity during pregnancy are linked to adverse health outcomes and lower quality of life (QoL). While physical activity (PA) and lifestyle support can improve well-being, effective interventions remain scarce. Supporting lifestyle change in pregnant mothers with obesity through the wearable internet-of-things (SLIM) intervention, developed in collaboration with maternity care professionals and pregnant women, integrates behavior change support and technology to promote weight management, PA and consequently QoL. OBJECTIVE The aim was to assess the effectiveness of the SLIM intervention in terms of improving QoL of pregnant women with overweight. Another aim was to describe PA of the participants during the intervention. METHODS This prospective quasi-experimental trial conducted at the maternity clinics evaluated the SLIM intervention, which combined core components of health technology, motivational interviewing, feedback, and goal setting. The intervention started from gestational week ≤20 and continued to 12 weeks after childbirth. The outcomes assessed were QoL (WHOQOL-BREF instrument) and PA (activity parameters gathered with Oura ring). QoL was assessed three times, at ≤20 gestational weeks (T0), at 34 gestational weeks (T1) and 12 weeks after childbirth (T2), and PA continuously during the intervention. Descriptive statistics were used to summarize demographic data, and hierarchical linear mixed-effects models for repeated measures were applied to assess changes in QoL domains across three timepoints, as well as their associations with background factors. Weekly mean MET variables were presented graphically with 95% confidence intervals (95% CI), and additionally, smart ring data were averaged for each three timepoints. RESULTS A total of 48 participants were included in the analysis. The mean total score of physical health domain of QoL increased between 34 gestational weeks and 12 weeks after childbirth 8.5 (95% CI 3.3, 13.7; p=0.002), but with other domains, scores decreased across all timepoints. PA decreased as pregnancy progressed, particularly for moderate-intensity activity (T0: Mean MET minutes per day 108.0; 95% CI 88.7, 127.4; T1: 65.9; 95% CI 53.7, 78.0). After delivery, PA increased and reached the same or even higher level than at the baseline (Mean moderate MET minutes per day T2: 118.9; 95% CI 92.2, 145.7). CONCLUSIONS Physical health-related QoL improved after childbirth, but psychological, social, and environmental domains remained unchanged. PA declined during pregnancy yet rebounded postpartum to baseline or higher levels. Findings suggest that while SLIM intervention may support physical recovery, comprehensive strategies addressing psychological well-being throughout pregnancy are needed. Further research with randomization and control group is needed to provide a deeper understanding of the effectiveness of SLIM intervention on QoL and PA. CLINICALTRIAL The trial was registered at the Clinicaltrials.gov register platform (ID NCT04826861) on 17 March 2021. INTERNATIONAL REGISTERED REPORT RR2-doi:10.1371/journal.pone.0279696