A new species of pseudothelphusid crab, Fredius ykaa, from the Rio Marau drainage, in the middle Amazon basin, state of Amazonas, Brazil, is described and illustrated. The new species is characterized by a first gonopod with a straight border between the cephalic spine and the apex, as well as by a highly reduced mesial lobe relative to the size of the cephalic spine. The new species is compared with four other species of the genus Fredius Pretzmann, 1967, occurring in the Brazilian Amazon region and its geographical distribution is discussed.