Objective To assess observed handwashing before eating among primary schoolchildren in a fragile setting in Palestine and examine how this behaviour is associated with household-level and guardian-related determinants. Design Cross-sectional baseline study embedded within a school-based cluster randomised controlled trial of a hand hygiene intervention. The trial is registered on ClinicalTrials.gov (NCT05964478), and this manuscript reports pre-results baseline findings alongside additional cross-sectional household survey data. Setting Public primary schools in rural and semiurban areas of the Hebron governorate, Palestine, Feb-ruary–March 2023. Participants Fifth and sixth grade schoolchildren enrolled in the trial and their co-resident guardians. Children were observed at school and guardians completed a questionnaire at home. Primary and secondary outcome measures The primary outcome was observed handwashing of both hands with water before eating at school, recorded by trained enumerators during a standardised snack break. Household-level and guardian-level determinants were derived from a structured questionnaire covering five domains: sociodemographic characteristics of guardian and child; household access to handwashing; guardian’s health and handwashing knowledge; guardian’s self-reported handwashing behaviour; and guardian-reported child handwashing at home. Associations between determinants and ob-served handwashing were estimated using generalised linear mixed-effects models with school as a random effect; effect modification by whether the responding guardian was the mother was explored. Results Observation and survey data were available for 931 child–guardian pairs across 26 schools. Overall, 37% of the fifth and sixth grade children were observed washing their hands before eat-ing at school, while 44% of guardians expressed high confidence that their child would do so and 84% exhibited good handwashing knowledge themselves. Poor guardian’s self-reported hand-washing behaviour was associated with a lower likelihood of children’s observed handwashing at school (adjusted OR (OR adj): 0.68; 95% CI 0.47 to 0.99), particularly when the guardian was the mother (OR adj: 0.55; 95% CI 0.40 to 0.90). Household water quality was perceived as poor by 95% of guardians. Poor perception was associated with lower likelihood of children washing hands at school (OR adj: 0.41; 95% CI 0.20 to 0.90). Conclusions In this fragile, water-insecure setting, low levels of observed handwashing before eating and the associations with guardian behaviours and household-level factors suggest that school-based handwashing promotion may be strengthened by complementary strategies that also engage households.