The marine planktonic diatom genera Rhizosolenia, Proboscia, Pseudosolenia and Neocalyptrella have been studied with emphasis on morphology and phylogeny. Material was collected October 2008—January 2011 at seven localities in the four provinces of the marine coastal waters of Thailand: Rayong, Chonburi, Chumphon and Phuket. Fifty strains were established in culture and used for morphological and phylogenetic analyses, complemented with morphological studies of field material. Morphological studies were done using light microscopy, scanning and transmission electron microscopy. Ten species and two varieties of Rhizosolenia were identified and described in detail: R. styliformis, R. acuminata, R. bergonii, R. clevei var. clevei, R. clevei var. communis, R. formosa, R. hyalina, R. imbricata, R. ostenfeldii, R. aff. pungens, R. setigera and R. loanicola sp. nov. Pseudosolenia calcaravis was also found, and detailed studies indicated that this taxon formed a species complex. Proboscia was represented by P. indica and P. siamensis sp. nov., but more species probably exist. Neocalyptrella was represented by N. robusta. The molecular studies supported Pseudosolenia, Proboscia and Neocalyptrella as separate genera. Culturing and molecular analyses were often difficult or unsuccessful. The data show that Rhizosolenia can be a monophyletic genus if R. pungens and R. setigera, which are distantly related to the remaining Rhizosolenia species, are described as a new genus. This will be done in a companion study of the type material of R. setigera.