A dinoflagellate isolated from a small rainwater rockpool in Finland was found to be a new species of Baldinia, here described as B. droopii sp. nov. This is only the third formally described Baldinia species and the second described by modern methods. The new species had a complement of generic characters that clearly affiliated it to Baldinia: presence of an internal honeycomb structure, termed a lamellar body, a ventral fibre associated with the longitudinal basal body and a pentagonal resting cyst; and the absence of an apical structure, thecal plates and trichocysts. The most distinctive species-specific characters were the presence of a resting cyst with tubiform processes, a paratabulation matching the pattern of the motile cell, and a life cycle exhibiting a diel rhythm, alternating between motile and non-motile coccoid cells. Motile cells occurred during the light period, peaking after 3-4 h of light period, while practically no motile cells occurred during the dark period. LSU rDNA sequences confirmed B. droopii as a new species, showing c. 14% sequence difference compared to B. anauniensis. Asexual reproduction occurred primarily by binary fission or via a so-called division cyst. Sexual reproduction occurred in the culture indicating that D. droopii is homothallic. The zygote may form a resistant cyst with tubiform processes, but there were indications that this stage may be by-passed. Different stages of the asexual and putative sexual reproduction were observed and documented by video recordings.