Abstract This study evaluates the influence of moisture sorption on the properties of flax fibres and their composites. The radial swelling of both the elementary and technical flax fibres at a wide range of relative humidity (RH) values is determined. Moreover, the effects of moisture sorption on both the thickness swelling and flexural properties of flax/polyester and flax/epoxy composites are evaluated. Park's model is used to define the fibre moisture sorption curve. The model suggests that the moisture is first sorbed on hydrophilic fibre surfaces or microcavities until specific sites within the fibre reach saturation and later, the moisture forms aggregates at high RH. Flax elementary fibres have a lower radial swelling coefficient when compared to technical fibres, with radial swelling coefficients of βr = 1.2 and 1.9, respectively. At the composite level, flax/polyester and flax/epoxy systems have approximately the same thickness swelling coefficients, βt = 0.5 and 0.6, respectively, over the RH range, 11–97%. This behaviour could be explained by the constraining effect of the resins on flax fibres due to higher stiffness of matrix and probably due to resin penetrating the fibre lumen. Flexural properties of longitudinal and transverse fibre-oriented flax/polyester and flax/epoxy composites conditioned at different RH, and tested in fully dry condition, decreased as RH increased beyond 54%, likely due to moisture swelling which induced damage. This study provides new insights into the moisture sorption and dimensional changes of both the elementary and technical flax fibres and their composites and also the effects of moisture on the flexural properties of the composites.