Recently, we reported a sulfonated hypercrosslinked polymer (SHCP-10) able to adsorb significant amounts of water from air even at <10% RH. We demonstrated that highly hygroscopic sulfonate groups are excellent candidates for enhancing the water sorption performance of hydrophobic carbonaceous materials, as our non-sulfonated reference exhibited negligible water uptake <70% RH. Here, we adapted this concept in a COF to exploit its crystalline nature and narrow pore size distribution. We describe a sulfonate-decorated COF possessing a steep uptake in the water sorption isotherm at <10% RH. The sulfonated COF is characterised extensively and the long-term stability and regeneration of the network is discussed.