We describe a transmission dark-field digital holographic microscope based on a Mach-Zehnder configuration for the detection of nanosize objects or objects smaller than the optical resolution limit. An optical stop adequately placed in the object beam removes the nondiffracted beam while keeping the light scattered by the object. This configuration combines an improved detection of objects smaller than the optical resolution with the refocusing capability yielded by digital holography. A theoretical analysis and an experimental demonstration are provided.