Background: This study investigates a simple, generic and easily scalable nudge to promote healthy and sustainable food choices at conferences by using a vegetarian lunch-default as a normative signal. Methods: At three conferences, participants registering electronically were randomized into two groups: Group 1 received a standard lunch registration presenting a non-vegetarian buffet as the default, but allowing the active choice of a vegetarian option; Group 2 received a registration presenting a vegetarian buffet as the default, allowing the active choice of a non-vegetarian option. The study also assessed gender differences for two of the conferences and the participants’ acceptance of the nudge at one of the conferences. Results: In experiment A the vegetarian choice increased from 2% to 87% (N = 108, P < 0.001). In experiment B it increased from 6% to 86% (N = 112, P < 0.001). In experiment C it increased from 12.5% to 89% (N = 110, P < 0.001). A significant tendency for men, but not women, to opt out of the vegetarian default was found and a clear majority of participants reported positive attitudes toward the nudge. Conclusions: Changing the lunch-default to a vegetarian option is an effective, generic, easy to scale and well-accepted nudge to promote healthy and sustainable food choices at conferences.