We showed a sulfonate-decorated covalent organic framework (COF-SO3H) capable of low RH atmospheric water harvesting. The network displayed an overall water capacity of up to 0.3 g•g -1 at 90% relative humidity (RH) and 25 °C, reaching almost 60% of its total uptake at ≤20% RH. Water uptake began <5% RH, representing a significant improvement in hydrophilicity compared to a non-sulfonated equivalent. Repeated water sorption-desorption cycles showed no significant decrease in uptake performance. In addition to retaining excellent AWH ability, COF-SO3H showed no measurable chemical change after many water adsorption-desorption cycles. Considering the low RH water uptake of COF-SO3H, we believe that the design of sulfonated networks could pave the way to high-performance AWH in extreme environments.