(1) Background: Implementing social-emotional learning (SEL) programs in Early Childhood Education (ECE) settings is a promising approach that can strengthen overall development and well-being during childhood and into adolescence and adulthood. This study described the development, implementation, and preliminary evaluation of a universal SEL program, i.e., Roundies, in the Finnish ECE context to address the need for professional development opportunities. (2) Methods: The Roundies program was a quasi-experimental pre-test/post-test with intervention and control design. A total of 194 children were assigned to either the intervention group (n = 136) or control group (n = 58) (MAGE = 60.35 months at pre-test; 48% boys). Teachers rated the children’s behaviors using the Strength and Difficulties Questionnaire (SDQ) at the pre-and post-test. (3) Results: Feedback collected by teachers on the program suggested that teachers were highly satisfied with the overall program and the components. Multilevel models showed significantly increased prosocial behaviors and reduced SDQ total difficulties in the intervention group compared to the control group. (4) Conclusions: These preliminary findings provide evidence of the effectiveness of the Roundies program in improving teachers’ capacity to support early SEL.