A base-promoted dearomative annulation between N-2-pyridylamidine and an atmospheric pressure of CO₂ was developed, affording a series of pyrido[1,2-a]-1,3,5-triazin-4-ones in moderate to excellent yields. CO₂ served as a carbonyl source, releasing H₂O as a solely clean byproduct. Moreover, no dehydrating reagent and transition-metal catalyst were required in this procedure. Thus, it represents a green, sustainable and straightforward pathway to access such frameworks.