Previous research has documented that exposure to green spaces has the beneficial effects of attention restoration and stress reduction. This study investigated the effects of indoor (classroom) and outdoor (green school garden) environments on attentional processes in interaction with emotion and physiological self-regulation. Children in third and fourth grades (n = 42) completed a school-related emotional Stroop task assessing the effects of outdoor and indoor classroom backgrounds when facing positive and negative stimuli. Children’s attentional patterns in a task completed in both environments were also assessed. Heart rate variability was registered at rest as an index of physiological self-regulation. The results revealed that children were less distracted from negative emotional materials when presented with outdoor compared with indoor background stimuli. Greater selective attention and sustained attention were shown in the green than in the classroom environment. Moreover, sustained attention varied in relation to physiological self-regulation but only when performing the task indoor.