We report a combined scanning tunneling microscopy and x-ray magnetic circular dichroism study to investigate the structural properties and magnetic behavior of a Co ultrathin film composed of dimer nanolines. These Co nanolines, ∼6 nm in length, are grown on a Si nanotemplate composed of nanoribbons self-organized on Ag(110). The first two Co layers present a weak magnetic response while upper Co layers exhibit an enhanced magnetization. Orbital and spin moments are experimentally determined. We show that an in-plane magnetization is favored and the magnetic anisotropy energy associated with the directions parallel and perpendicular to the nanolines are measured. The Co ultrathin film is shown to behave as a superparamagnetic system composed of one-dimensional segments containing each ∼170 ferromagnetically coupled Co atoms, with a blocking temperature estimated to be between 20 and 40 K. Our set of experiments allows for a comprehensive description of the magnetic behavior of the Co nanoline ultrathin film grown on a functionalized metallic substrate.