In this work, full triazine two-dimensional frameworks (2DC3N3) were synthesized and used to detect metal ions in aqueous solutions. 2DC3N3 showed excellent performance to sense Hg(II) with high selectivity (>96%) and was designed as a recyclable sensor for Hg(II) detection. 2DC3N3 was synthesized using catalyst-free [2+2+2] cyclotrimerization of sodium cyanide and cyanuric chloride at ambient conditions. The as-prepared 2DC3N3 sheets with several hundred micrometers lateral size exhibited strong excitation-dependent fluorescence with 63% quantum yield and maximum emission at 428 nm. The emission of 2DC3N3 decreased with 14.74 × 103 L·mol–1 quenching constant (KSV) upon interaction with Hg2+ ions. This effect was used as a strong signal to detect Hg(II) with a 0.98 nM detection limit. The high Hg2+ removal capacity of 2DC3N3 was attributed to cooperative interactions of nitrogen atoms of 2DC3N3 pores and Hg2+, as suggested by computational studies. Taking advantage of the straightforward synthesis of 2DC3N3 and its selectivity, it can be used as an efficient platform for monitoring Hg2+ in waste and drinking water.