Many studies highlight the role of mixed versus monospecific forests to provide numerous ecosystem services. Most reports of the positive effects of tree mixture on biodiversity focus on coniferous–deciduous combinations, but little is known about the effects of mixtures combining two coniferous tree species. We assessed the effects of mixed versus monospecific stands of Pinus sylvestris and P. pinaster on the understory richness and composition and its relationship with the soil status, based on research with six triplets in northern Spain. In ten square meter quadrats randomly located per plot, the cover of every understory vascular plant species was estimated visually and data were codified according to Raunkiær’s life-forms. One soil pit of 50 cm depth was dug in each plot to determine the soil water (water holding capacity) and fertility carbon and exchangeable cations stocks) status. A water-stress gradient associated with the overstory composition indicated that P. pinaster tolerates lower soil water content than P. sylvestris. Mixed stands are under greater water stress than monospecific P. sylvestris stands but maintain the same level of understory richness. Also, a soil fertility gradient defined by organic carbon and exchangeable magnesium stocks was identified. Hemicryptophytes, whose abundance is greater in mixed stands, were the only understory life-form positively correlated to soil fertility. We conclude that the mixture of both Pinus species should continue to be favored in the study area because it helps to maintain understory richness under greater water-stress conditions and improves soil fertility.