The benefits of taking up physical activity are well established and social support has been identified as one of the main determinants of this behavior. To investigate the association between social support and weekly frequency of physical activity in adults working at a public university in the state of Rio de Janeiro, Brazil. This is a cross-sectional population study with a convenience sample of 189 contract workers of both sexes, aged from 21 to 72 years (39.00 ± 11.43). The instruments employed were the short version of the International Physical Activity Questionnaire and the Social Support for Physical Activities Scale. Fisher's exact test was used to estimate the distribution of physical activity frequency. Poisson regression was used for association analyses. The significance level was set at 5%. A significant association was detected between social support and weekly physical activity frequency (p < 0.05). Social support for physical activity of moderate or vigorous intensity was associated with both weekly frequency of walking (odds ratio [OR]: 1.32; 95% confidence interval [95%CI]: 1.11-1.58) and weekly frequency of vigorous physical activity (OR: 1.34; 95%CI: 1.08-1.67). Additionally, people who reported receiving social support for walking were more likely to have increased weekly frequency of walking (OR: 1.22; 95%CI: 1.00-1.49). Social support for physical activity from relatives and friends is associated with weekly physical activity frequency. Notwithstanding, this association was stronger for weekly frequency of vigorous intensity physical activity.