Background and Objective: Assertiveness is a crucial skill in the nursing profession to reinforce interprofessional relationships, prevent workplace violence, reduce work stress, improve professional efficacy, develop leadership skills, autonomy and job satisfaction. Assertiveness when inculcated among nursing students will immensely contribute to their professional development and a sustainable career. Since no standard tools are available to measure assertiveness among Sinhala speaking students, this study is aimed to cross-culturally adapt the Sinhala version of Begley and Glacken’s Assertiveness Scale among nursing students in Sri Lanka. Methods: Permission to translate, cross-culturally adapt and use the 28-item Begley and Glackens’ Assertiveness Scale were obtained from its developers. The content and consensual validity of translated version was assessed with a two round Delphi process using five experts and a consensus evaluation. If 70% or more of the ratings for an item are in categories 0-3 the item was removed or reworded. The process was repeated for the reworded items and those were retained if 70% or more of the ratings were in categories 4-6 and /or 7-9. Subsequently, Content Validity Index (CVI) was evaluated using I-CVI, Universal Agreement (S-CVI/UA) and Average CVI (S-CVI/Ave). Finally, the scale’s reliability was evaluated using Cronbach’s alpha in a randomized sample of 140 nursing undergraduates from the University of Colombo. Results: All the items in category 4-6 and 7-9 were retained. Sinhala version of the assertiveness scale showed maximum CVI of all individual items (I-CVI=1.0) and maximum overall CVI (S-CVI/UA = 1.0; S-CVI/Ave = 1.0). The reliability analysis indicated Cronbach’s alpha of 0.753. Conclusions: The Sinhalese version of the Begley and Glacken’s assertiveness scale is a culturally adapted, valid and reliable instrument to measure assertiveness among nursing students.