Sea-level rise is expected to cause saltwater intrusion into tidal freshwater wetlands. The resulting changes in the soil environment would likely cause shifts in plant and microbial communities and alter ecosystem functions. To simulate saltwater intrusion, we constructed a solar-powered automated irrigation system. The system consisted of holding tanks that were filled with freshwater or artificial brackish water during high tide. During low tide, the water was gravity-fed through solenoid valves and metered out to each plot. This system was controlled by an open-source microcontroller platform and built from off-the-shelf electronics at a cost of $1800. Porewater salinity in near-surface soils of the plots receiving brackish water was increased from freshwater to oligohaline levels within one month of operation and was maintained throughout the first summer of operation. This porewater manipulation led to changes in the plant community and in exchanges of carbon dioxide and methane between the marsh and atmosphere. The system has proven to be a reliable way of manipulating wetland salinity for studying changes in the physicochemical environment. With modifications, it could be used to manipulate nutrient loads, hydrology, and sediment supply in other wetland systems.