Herein, the physicochemical properties and enhanced adsorption capacity of highly porous thin-layer graphitic carbon nitride (g-C3N4) nanosheets (CNNs) in relation to the separation of dyes are investigated. Cationic and anionic dyes with similar sizes are adopted to study the adsorption and separation properties of graphitic carbon nitride. Highly porous g-C3N4 nanosheets were synthesized via the direct thermal polycondensation of melamine followed by a thermal exfoliation. A comparative study of bulk g-C3N4 and porous thin-film g-C3N4 nanosheets was carried out. The results revealed the formation of highly porous g-C3N4 nanosheets with a well-oriented structure and adequate chemical stability. Porous thin-layer g-C3N4 nanosheets possess a high surface area of 212 m2/g as compared to 10 m2/g of the bulk material. Interestingly, adsorption experiments employing both cationic (methylene blue, rhodamine 6G, and rhodamine B) and anionic (methyl orange, eosin Y) dyes, as well as their mixtures, revealed that g-C3N4 nanosheets exhibited excellent selective adsorption capacity toward cationic dyes, which is followed by very short equilibrium times (e.g., 100% adsorption of MB within 10 min). The present findings can be well interpreted in terms of improved textural and structural properties of CNNs in conjunction to the dispersion and electrostatic interactions between the different dyes and the surface of CNNs. The experimental findings are further corroborated by means of periodic self-consistent charge density functional tight binding theoretical calculations.