Microalgae of the division Haptophyta are a major component of the marine nanoplankton and present in all seas. They are important primary producers and grazers of picoplankton in the ocean, and their metabolic products can have an impact on global climate. Due to their small size, species are difficult to distinguish by microscopy, and knowledge on their diversity and distribution is incomplete. Environmental sequencing studies have revealed a high marine protist diversity. We review the current knowledge on diversity and distribution of haptophytes revealed by Sanger sequencing of clone libraries (environmental sequencing) and by high-throughput sequencing of amplicons (metabarcoding). We also discuss the methodology used. Finally, we provide a curated haptophyte reference 18S rRNA-gene database for future taxonomic assignment of environmental sequences and metabarcoding reads.