This study sought to identify the psychosocial factors that influence handwashing behaviour and to design behaviour change techniques applying the risk, attitude, norm, ability, and self-regulation (RANAS) behaviour change approach in Rohingya camps in Cox's Bazar, Bangladesh. This was a cross-sectional study conducted in four Rohingya camps located in Ukhiya and Teknaf sub-districts. Participants were selected by systematic random sampling. A structured interviewer-administered questionnaire was used. Analysis of variance testing (ANOVA) was used to determine the significant predicting variables. A total of 1,358 individuals participated in the interview. The mean age was 31.11 ranging from 13 to 75 years. The majority were female (84.9%). Around 75% of participants reported practising handwashing. Several psychosocial predictors were identified for handwashing behaviour with medium to large effect size such as health knowledge, beliefs about costs and benefits, feelings, social norms, abilities, and self-regulation factors. Significant predictors were additionally identified in individual camps. The behaviour change techniques included providing information, demonstration of handwashing, providing rewards, evoking emotions, and public commitments amongst others proposed in this study. Identified psychosocial determinants should be considered while promoting sustainable handwashing behaviour along with adequate supplies and infrastructure. This study also suggests evaluating the proposed behaviour change interventions considering the contextual factors related to handwashing.