Exposure to green spaces can reduce the negative effects of stress. This study examines how frequency of visits and time spent in urban green spaces (UGS) affect urban dwellers’ subjective well-being. We also investigated the numbers of respondents visiting UGS, their primary motivation, and constraints on their ability to visit. Using quota sampling, an online survey was conducted of 400 residents of Daejeon City, South Korea. ANOVA results indicated no significant interactions between visit frequency and time spent in UGS. Respondents who had visited UGS within the past two weeks expressed higher positive and lower negative emotions than did non-visitors, regardless of visit frequency, and regular visitors showed higher general life satisfaction levels. These positive effects were confirmed by estimated structural equation models. However, the time spent in UGS did not affect emotions or life satisfaction in general. Heavy users mostly visited UGS to walk, and light/non-users cited the lack of urban green spaces near their home as the major constraint on visiting UGS. The estimated structural equation models clearly show positive effects from motivation and negative effects of constraints and access time to UGS on visit frequency. To improve urban dwellers’ subjective well-being, UGS should prioritize good walking environments and accessibility.