Can states usher in more peaceful relations with their neighbors by signing agreements that delineate their territorial boundaries? Theory suggests such a possibility, but the empirical evidence to date remains limited by research design and variable measurement decisions. After assembling a new data set on international boundary agreements, the current study conducts the first thorough test of this question during the period 1816–2001. The findings indicate that once neighboring states settle their borders, they are less likely to go to war or experience militarized interstate disputes with one another. These pacific effects persist across numerous time periods even after controlling for joint democracy, a characteristic that both theory and this analysis show to be positively related to settled borders. Through these findings, the study suggests that signing international boundary agreements can bring neighbors a more peaceful relationship with one another, regardless of the characteristics of their respective governmental regimes.