The invention provides seed and plants of the spinach line designated SMB66-1086M. The invention thus relates to the plants, seeds and tissue cultures of spinach line SMB66-1086M, and to methods for producing a spinach plant produced by crossing a plant of spinach line SMB66-1086M with itself or with another spinach plant, such as a plant of another line. The invention further relates to seeds and plants produced by such crossing. The invention further relates to parts of a plant of spinach line SMB66-1086M, including the fruit and gametes of such plants.