Online courses are increasingly used to scale up professional learning. However, engaging people throughout a course and encouraging them to take actions afterward is challenging, especially with culturally diverse participants whose response to encouragement may vary. Social norms can serve as a strong encouragement to participants, but they are mostly unobservable in online courses. We therefore tested how communicating three established kinds of social norms impacts behavioral outcomes and how the effects vary across different cultures. Findings from a randomized controlled field experiment in a Massive Open Online Course entitled Nature Education show that both the type of norm message (descriptive; dynamic; injunctive) and the cultural context (China; US) influence how the intervention improves course outcomes: US participants' completion rate rose 40% following an injunctive norm message stating that participants ought to conduct nature education, while other norm messages were ineffective for both US and Chinese participants. Regardless of the social norm message, nearly all participants engaged in nature education activities within three months after the course ended. Communicating social norms can impact participants' behavior in the course and beyond, which can offer a strategy for supporting online professional learning and tailoring messages to different cultural contexts.