We study how social security influences joint retirement of couples. We exploit three decades of administrative data from Denmark to explore joint retirement in two complementary settings. In the first setting, we exploit the discontinuous increase in retirement observed when individuals become eligible for public pension benefits to identify the causal effects on their spouses. We find that spouses are more likely to retire right when their partners reach pension eligibility age, with a spillover effect across spouses of 7.5%. We further unpack this result by studying additional margins of adjustment such as benefit claiming and earnings, and by documenting meaningful response heterogeneity. We find age differences within couples to be a crucial determinant of joint retirement, which is primarily driven by older spouses who continue to work until their younger partners reach pension eligibility. Controlling for these age differences uncovers a gender gap where female spouses are more likely to adjust their behavior to retire jointly, and this gap remains after controlling for earnings shares within couples. In the second setting, we study to what extent couples adapt their behavior to retire jointly after a reform increases pension eligibility ages. We find spillover effects across spouses comparable to those from the first setting, in which eligibility ages were stable and known by couples well in advance. This suggests that spouses do not face adjustment costs limiting their capacity to retire together after the reform.