Many groundwater aquifers in important agricultural areas are exploited beyond their sustainable limits. Groundwater overuse can reduce streamflow across political boundaries, leading to transboundary management challenges. Although conflicts over transboundary water resources do arise, these conflicts can also prompt improved aquifer management. Portions of the Republican River Basin, which overlies the High Plains Aquifer in the central United States, have been under court-ordered groundwater restrictions to meet interstate streamflow requirements since 2004, following the 2002 Kansas v. Nebraska and Colorado Supreme Court case. We examined the impacts of these restrictions on groundwater levels, pumping volume, agricultural productivity, and streamflow in the Nebraska portion of the basin to assess how transboundary agreements can affect groundwater sustainability in agricultural systems. We synthesized available data for 1990-2014 to analyze trends before and after restrictions went into effect in 2004. After controlling for climate covariates, we found that restrictions reduced pumping volumes in the study area, resulting in increased streamflow across the Nebraska-Kansas border. Furthermore, restrictions appear to have reversed the declining trend in groundwater storage. Notably, this reversal contrasts with continuing decline in the unrestricted Kansas portion of the basin, suggesting the court-ordered restrictions have altered the sustainability trajectory of this region. The impacts of pumping restrictions on regional agricultural yields and productivity are examined. Our analysis of this system suggests that by setting external limits on resource use, enforceable transboundary water agreements can stimulate sustainable groundwater management and counter local incentives for overextraction.