Uncithrombium modestum (Berlese, 1888) is redescribed basing on the type specimen and three other individuals of type series allocated on four slides from Berlese Acaroteca, deposited in Firenze, Italy. The holotype and one individual of the type series are illustrated, measured and described. Diagnosis and description are supplemented with standard measurements, not considered in original publication. Differentiating diagnosis of U. modestum refers to four Neotropical species of Uncithrombium Gabryś, 1999 known from postlarval forms and satisfactorily described in literature: U. modestum, U. manriquei, U. panamense, and U. longipes. Concise and updated diagnosis of the genus Uncithrombium Gabryś, 1999 and the key to all known species are provided as well.