In 2022, Portugal ranked among the 10 countries with the highest prevalence of insufficient physical activity (PA) levels, exceeding 50%. This study aimed to characterize epidemiological studies conducted in Portugal that assessed population PA and evaluate their quality based on the characteristics of a PA surveillance system (SS). Studies that analyzed data on PA, covering at least mainland Portugal, were selected and characterized. An original quality framework was developed as part of this study to assess whether the studies can be considered SS: 1) use a scientifically validated instrument; 2) repeated measurements; 3) assess age groups representing a lifespan; 4) sample representative of Portugal; 5) combine self-reported data with accelerometry; 6) assess PA contexts; 7) collect sedentary behavior data; 8) use the World Health Organization guidelines as a cut-off for sufficient PA; 9) interpretation of results based on sociodemographic variables; 10) accessibility and dissemination of results. Ten studies were characterized. When submitted to the framework, compliance with the characteristics ranged from 50% to 90%. A high number of studies assess PA levels in Portugal but employ different methodologies, leading to difficult-to-compare results. The lack of full compliance with the criteria emphasizes the limitations of monitoring. These findings highlight the need for the implementation of a unified SS in Portugal.