Introduction: Nomophobia, the fear of lack of access to electronic communication devices, is an emerging psychosocial problem among youth. Nomophobia Questionnaire(NMP-Q) is the most widely used instrument to assess nomophobia, however, there is limited evidence on its validity in South Asia. Objective: To translate, culturally-adapt and validate NMP-Q for the use among Sri Lankan adolescents. Methods: NMP-Q was translated from English into Sinhala, using standard protocols. Content validity of the self-administered questionnaire, was assessed by a panel of experts and it was validated on a sample of 181 Sinhala-speaking grade 10-12 students. Confirmatory Factor Analysis (CFA), Multi-trait Scaling Analysis (MSA) and convergent validity using known group comparison were conducted. Cronbach’s alpha coefficient was used to assess reliability . Results: 51.9% were males. Mean age was 15.9 years. Everyone used one or more electronic device with 177(97 .8%) using mobile phones and 81 (44.8%) owned one. CFA showed that the original four-factor model (not being able to communicate, losing connectedness, not being able to access information, and giving up convenience) showed best fit to the data (RMSEA 0.089, SRMR 0.069, GFI 0.82, AGFI 0.77 and NNFI, CFI and IFI >0.9), and MSA supported it. Significant difference in certain behaviours were shown by NMP-Q scores; uses only when necessary(64.6vs73.2, p=0.024), keep nearby when sleeping(65.5vs76.6, p=0.005), carry a charger(79.9vs66.1, p=0.004), tried to reduce device use(66.1vs78.5, p=0.007). Cronbach’s alpha>0.75. Conclusions: This study provides evidence that the Sinhala version of the NMP-Q is a valid and reliable instrument to assess nomophobia in Sri Lankan school-going adolescents.