This study examines how different forms of U.S. retrenchment shape South Korean public attitudes toward security burden-sharing amid persistent nuclear threats from North Korea. While retrenchment has emerged as a recurrent feature of U.S. foreign policy since the Trump administration—ranging from force downsizing and alliance delegation to strategic abandonment—its implications for allied perceptions and policy preferences remain underexplored. Using a large-N survey experiment conducted with 3,450 South Korean respondents, this research presents randomized treatments depicting alternative types of U.S. retrenchment following a vignette describing North Korea’s advancing nuclear and missile capabilities. The results reveal that Koreans perceive greater threat and demonstrate higher willingness to share defense burdens when retrenchment takes the form of delegating defense responsibilities to allies, rather than simple downsizing. Conversely, strategic asset withdrawal (abandonment) generates the strongest support for independent nuclear armament, especially among older, conservative respondents. Moreover, while greater trust in the United States does not necessarily reduce willingness to contribute more to defense, high threat perception toward North Korea significantly amplifies support for both burden-sharing and alliance autonomy. These findings suggest that the political effects of U.S. retrenchment depend less on its occurrence than on how it is operationalized and framed—highlighting the need for nuanced alliance management strategies under conditions of dual threat and strategic uncertainty.