Children's access to green spaces in urban areas is highly associated with their physical and psychological wellbeing. In the last two decades, children's access to urban outdoors has reduced, and concerns about children's physical health have increased. The number of overweight and obese children has considerably risen from both developing and developed countries. Turkey is a developing country, where studies about childhood obesity and its relationship with access and use to urban green spaces are limited. Therefore, this study first investigates the relationship between children's visits to urban green spaces and age, gender, proximity, quality and physical health; Second, it also investigates obesity prevalence in school children (aged six to eighteen) and identifies the relationship between physical fitness and availability of green spaces close to children's living arrangements, visits to green spaces, regular physical activity in green spaces and socioeconomic status (SES). This study includes 2668 student from 24 state schools in 4 different districts of Istanbul. In each region, two schools from each education level (primary, secondary and high school) were retrieved. Physical fitness indicators (weight and height) were measured, and children were asked to take part in questionnaires about their green space use. This study identified that children were more likely to visit green areas, if they were existed within proximity to their homes and quality of urban green spaces was an essential factor for visits. Visiting green spaces were significantly correlated with age, and younger children were more likely to visit compared to older children. A quarter of children (25.67 %) were overweight or obese. Gender [0.82; p=0.038), and age [Age<10 was 2.06; p= 0.000 and Age 10-14 was 1.93; p = 0.000) were statistically significant variables for overweight and obesity. It was identified that odds of children who lived in gated communities, being overweight or obese were 29 % [1.29; p=0,048] higher then children living in traditional neighbourhoods. Children visiting urban green spaces for physical activity were 29 % [0.71; p=0.032) less likely to be under the risk of being overweight or obese. The study identified obesity prevalence is higher in younger children, which might indicate a growing number of overweight and obese children and needs monitoring in the future.