A series of potent and selective [1,2,4]triazolo[1,5-a]pyrimidine PDE2a inhibitors is reported. The design and improvement of the binding properties of this series was achieved using X-ray crystal structures in conjunction with careful analysis of electronic and structural requirements for the PDE2a enzyme. One of the lead compounds, compound 27 (DNS-8254), was identified as a potent and highly selective PDE2a enzyme inhibitor with favorable rat pharmacokinetic properties. Interestingly, the increased potency of compound 27 was facilitated by the formation of a halogen bond with the oxygen of Tyr827 present in the PDE2a active site. In vivo, compound 27 demonstrated significant memory enhancing effects in a rat model of novel object recognition. Taken together, these data suggest that compound 27 may be a useful tool to explore the pharmacology of selective PDE2a inhibition.