We combined time-series from a monitoring program and specific field observations in order to: 1) describe the characteristics of thin layers of phytoplankton (TLP) in the Galician Rías Baixas (NW Iberian Peninsula), 2) investigate the relationship between spatially-extended TLP (se-TLP) events and environmental variables, and 3) analyze the relationship between TLP occurrence and cell densities of toxin-producing harmful algal bloom (THAB) species. Between 2012 and 2015, a total of 118 TLP were detected in any of the 39 stations weekly sampled in the study area in the frame of the monitoring program, which represents a frequency of occurrence of 2%. Most TLP (84%) were detected between May-August and located slightly below shallow pycnoclines. Eight se-TLP events, i.e., when at least five stations were simultaneously influenced by the occurrence of a TLP, were identified during the period 2012–2015. Six out eight of these events happened during summer-upwelling conditions, and according to a Principal Component Analysis (PCA) they were positively correlated with thermal stratification. The other two events, detected during spring-downwelling, were positively correlated with haline stratification. A logistic regression model including six variables (surface temperature, relative humidity, bottom salinity, a proxy for the intrusion of the Miño River plume into Ría de Vigo, Miño River flow, and bottom temperature) explained 42% of the variance in the occurrence of se-TLP events. The temporal persistence of TLP events was confirmed by observations carried out during specific cruises, which showed how TLP were formed and disappeared over short periods of time in response to changes in vertical mixing. TLP were more frequently observed in Ría de Pontevedra, which was characterized by longer harvesting closures due to detection of Dinophysis toxins (DSP: diarrhetic shellfish poisoning toxins and pectenotoxins) above regulatory levels. About 25% of the TLP detected in the Rías Baixas were associated with increased cell densities of potentially toxic Pseudo-nitzschia species (ASP: amnesic shellfish poisoning) and of D. acuminata. These results suggest that a relationship may be established between the occurrence of TLP and the growth and/or accumulation of the main THAB species in the Galician Rías Baixas.