Integrating a wettability-patterned surface into a multiscale micro/nanostructured evaporator has proven to enhance the thermal performance of an ultrathin vapor chamber (UTVC), but the mechanism of the enhancement is unknown. To reveal the effect of the wettability-patterned multiscale evaporator on the thermal performance, the hydrophobic area fraction of the wettability-patterned surface in this study varies from 6.25% to 66.9%. The inner surface of the evaporator substrate is chemically patterned to form the wettability-patterned evaporator. The evaporator and a condenser with a supporting micropillar array comprise the UTVC with a less than 0.25 mm thickness. The thermal performance of UTVCs with various hydrophobic area fractions is measured under the optimum charge ratio. Specifically, the optimum hydrophobic area fraction is found at around 34%. Besides, a visualization experiment is conducted on the evaporation of a wettability-patterned glass substrate covered with a wire mesh to help figure out the mechanism of the enhancement. It is found that the wettability-patterned surface elongates the triple contact line, enhancing the evaporation rate. Based on its effect on the evaporation and the thermal resistance network of a vapor chamber, an evaporation model of the wettability-patterned evaporator in the UTVC is proposed. The model predicts the optimum hydrophobic area fraction is 29.9%, close to the experimental results. Moreover, it also well estimates the thermal performance of UTVCs with wettability-patterned evaporators at various hydrophobic area fractions.