Background: Objective walkability is an important correlate of adults’ physical activity. Studies investigating the relation between walkability and children’s physical activity are scarce. However, in order to develop effective environmental interventions, a profound investigation of this relation is needed in all age groups. The aim of this study was to investigate the association between objective walkability and different domains of children’s physical activity, and to investigate the moderating effect of neighborhood socio-economic status in this relation. Methods: Data were collected between December 2011 and May 2013 as part of the Belgian Environmental Physical Activity Study in children. Children (9–12 years old; n = 606) were recruited from 18 elementary schools in Ghent (Belgium). Children together with one of their parents completed the Flemish Physical Activity Questionnaire and wore an accelerometer for 7 consecutive days. Children’s neighborhood walkability was calculated using geographical information systems. Multilevel cross-classified modeling was used to determine the relationship between children’s PA and objectively measured walkability and the moderating effect of neighborhood SES in this relation. Results: In low SES neighborhoods walkability was positively related to walking for transportation during leisure time (β = 0.381 ± 0.124; 95% CI = 0.138, 0.624) and was negatively related to sports during leisure time (β = −0.245 ± 0.121; 95% CI = −0.482, −0.008). In high socio-economic status neighborhoods, walkability was unrelated to children’s physical activity. No relations of neighborhood walkability and neighborhood socio-economic status with cycling during leisure time, active commuting to school and objectively measured moderate- to vigorous-intensity physical activity were found. Conclusions: No univocal relation between neighborhood walkability and physical activity was found in 9–12 year old children. Results from international adult studies cannot be generalized to children. There ...