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The safest protection from UV radiation (UVR) exposure is offered by clothing and its protectiveness depends on fabric composition (natural, artificial or synthetic fibers), fabric parameters (porosity, weight and thickness) and dyeing (natural or synthetic dyes, dye concentration, UV absorbing properties, etc.). In this study the UV protection properties of two fabrics made of natural fibers (flax and hemp) dyed with some of the most common natural dyes were investigated. UVR transmittance of fabrics was measured by two methods: one based on the utilization of a spectrophotometer equipped with an integrating sphere (in vitro test), and the other based on outdoor measurements taken by a spectroradiometer. Transmittance measurements were used to calculate the ultraviolet protection factor (UPF). Experimental results revealed that natural dyes could confer good UV protection, depending mainly on their different UVR-absorbing properties, provided that the fabric construction already guaranteed good cover. An increase in cover factor caused by the dyeing process was also detected. Weld-dyed fabrics gave the highest protection level. The comparison between the two methods applied to measure fabric transmittance pointed out that the UPFs calculated by in vitro measurements were generally lower than those based on outdoor data, indicating an underestimation of the actual protection level of tested fabrics assessed by the in vitro test.