Psammococconeis and Amphicocconeis are recently created diatom genera showing a high morphological plasticity-variability, making particularly difficult identification only based on morphology. The sternum valve (SV) of Amphicocconeis is very similar to that of Cocconeis, whereas its raphe valve (RV) is highly dissimilar between the two genera. The RV in Amphicocconeis has curved terminal raphe fissures and striae composed of macroareolae with more or less complex arrangement, unique to each taxon. The Amphicocconeis valvocopulae are most of the time extended, with pores arranged along rows, or clusters of pores, and are an important taxonomic criterion. Several Amphicocconeis were originally described as Cocconeis and thereafter renamed after accurate examination with scanning electron microscopy (SEM). The number of described Amphicocconeis is low, while numerous species probably remain to be described, named or renamed. Several tropical Amphicocconeis from two Polynesian archipelagos are here investigated, from which three are described as new. Psammococconeis is a genus established prior to Amphicocconeis, originally described with simple terminal raphe endings, while their SV has characteristics close to Amphicocconeis. Psammococconeis is here re-defined as an invalid genus, after re-examination of the Brazilian type material of P. brasiliensis M.Garcia (renamed as Amphicocconeis brasilensis comb. nov.), and a new Amphicocconeis is described from this Brazilian material.