Decades of oceanographic observations on the Mediterranean Sea have consistently indicated that its open-sea region presents oligotrophic characteristics during long periods of the year (from late spring to autumn) with very low and uniform surface chlorophyll values interrupted by the presence of several productivity hot-spots associated to mesoscale features. Another commonly observed phenomenon in the basin is the widespread and consistent presence of subsurface or deep phytoplankton biomass accumulations. However, given the difficulty of observing those deep structures with the adequate spatial and temporal resolution, a comprehensive description of their spatial and temporal variability is yet to be done. Here we use a 3D hydrodynamic-biogeochemical coupled model of the entire Mediterranean Sea to explore the contribution of the deep chlorophyll maximum (DCM) to total plankton biomass and primary productivity in the open-sea regions of the basin. We found that, on annual average, DCM are present in 73.5 % of total open-sea regions with a clear annual cycle, being more frequent (up to 98% of the area) in summer months and less common (~9%) during winter. Our model also indicates that, on average, nearly 53% of total phytoplankton biomass and ~62% of total primary production in open-sea regions of the Mediterranean takes place within such structures. They must, then, be duly considered for a correct assessment of the biological productivity in the open sea regions of the Mediterranean basin.