Greening schoolyards is an initiative to reconnect children with nature and afford meaningful experiences that foster children's well-being. To strengthen the empirical basis for greening schoolyards, we conducted a longitudinal prospective intervention study with a two-year follow-up, to investigate the impact of greening schoolyards on schoolchildren's (age 7–11) appreciation of the schoolyard, and their physical, cognitive, and social-emotional well-being. Data were collected amongst nine elementary schools in moderate-to-high-urbanized areas in The Netherlands with approximately 700 children at each measurement. At baseline, all nine schoolyards were paved. Five schools greened their schoolyard between baseline and first-follow-up. Objective measurements included accelero-based measurements of physical activity during recess, attentional tests (Digit Letter Substitution Test, Natu & Argwal, 1995; Sky Search Task, Manly et al., 2001) and a social orientation test (Social Orientation Choice Card, Knight, 1981). Self-report questionnaires included children's appreciation of the schoolyard (naturalness, likability, attractiveness and perceived restoration), and their social- and emotional well-being (Strength and Difficulties Questionnaire, van Widenfelt, Goedhart, Treffers & Goodman, 2003; Social Support, RIVM, 2005; Pediatric Quality of Life Inventory, Varni, Seid & Kurtin, 2001). Multilevel data analyses support our expectation that greening has a positive impact on children's appreciation of the schoolyard, their attentional restoration after recess and social well-being. Furthermore, our results indicate that greening stimulates physical activity of girls. We found no impact on emotional well-being. These findings provide some support for the relevance of greening schoolyards and may guide further development of schoolyards that facilitate the well-being of schoolchildren.