Stimuli-responsive　photoluminescent　(PL)　materials　have　been　widely　used　as　fluorescent　ink　for　data　security　applications.　However,　traditional　fluorescent　inks　are　limited　in　maintaining　the　secrecy　of　information　because　the　inks　are　usually　visible　by　naked　eyes　either　under　ambient　light　or　UV-light　illumination.　Here,　we　introduced　metal-free　water-soluble　graphitic　carbon　nitride　quantum　dots　(g-CNQDs)　as　invisible　security　ink　for　information　coding,　encryption,　and　decryption.　The　information　written　by　the　g-CNQDs　is　invisible　in　ambient　light　and　UV　light,　but　it　can　be　readable　by　a　fluorescence　microplate　reader.　Moreover,　the　information　can　be　encrypted　and　decrypted　by　using　oxalic　acid　and　sodium　bicarbonate　as　encryption　reagent　and　decryption　reagent,　respectively.　Our　findings　provide　new　opportunities　for　high-level　information　coding　and　protection　by　using　water-soluble　g-CNQDs　as　invisible　security　ink.　Secret　communication:　The　information　written　by　using　carbon　nitride　dots　as　security　ink　is　invisible　under　illumination　with　both　ambient　light　and　UV　light.　The　encoded　information　is　readable　by　a　fluorescence　microplate　reader,　and　it　can　be　further　encrypted　and　decrypted　by　quenching　and　recovering　the　fluorescence　of　the　security　ink　using　H2CO4　and　NaHCO3,　respectively.　©　2016　Wiley-VCH　Verlag　GmbH　&　Co.　KGaA.