In this work, as a proof of principle, the design and performance evaluation of a simple, cheap and efficient colorimetric test for the detection of the NS1 protein of dengue virus, assisted by an immunoconjugate of magnetite (Fe3O4) nanoparticles coupled to anti-NS1 antibodies is reported. A monoclonal antibody against the NS1 antigen was covalently immobilized on the surface of superparamagnetic iron oxide nanoparticles (SPIONs ∼ 20 nm) and used for the immunodetection of this protein. When the magnetic immuno-nanoplatform is added into infected serum, it conjugates with the NS1 protein and can then be easily separated using an external magnetic field; then, the recovered immunoconjugate is transferred into a well containing a second immobilized NS1-antibody to form an ELISA-type system. When the NS1 protein is present, a color change to blue is induced by reaction with the Perls reagent, which is consistent with the formation of a SPION-antibody-NS1 antigen-antibody conjugate that confirms infection. No false positives were found when NS1 was not present or a different antibody and the NS1 protein were added into the system. The experimental findings could be extrapolated and scaled up to lead to future developments of simple, quick, and inexpensive, in situ biomolecular diagnostic tests for emergent viral infections.