How large is the gender pay gap among university professors, and how do institutional pay-setting mechanisms shape this disparity? This paper provides novel empirical evidence on the gender pay gap among professors at a renowned German university. Using detailed human resources data for the time span 2013 to 2021, we document a statistically significant conditional gender pay gap in professorial salaries of 5.2%, after controlling for employment characteristics, socio-demographics, performance measures, and faculty and year fixed effects. Our findings show that these differentials can be attributed mainly to lower returns from retention negotiations, which have a particularly strong impact during the earlier stages of academic careers. These results highlight the importance of pay system designs in promoting gender equity in academia.