Development of methods allowing determination of even ultralow levels of immunoglobulins in various clinical samples including whole human blood and plasma is a particular scientific challenge, especially due to many essential discoveries in the fields of immunology and medicine in the past few decades. The determination of IgG is usually performed using an enzymatic approach, followed by colorimetric or fluorimetric detection. However, limitations of these methods relate to their complicated setup and stringent requirements concerning the sample purity. Here, we present a novel approach based on magnetically assisted surface enhanced Raman spectroscopy (MA/SERS), which utilizes a Fe3O4@Ag@streptavidin@anti-IgG nanocomposite with strong magnetic properties and an efficient SERS enhancement factor conferred by the Fe3O4 particles and silver nanoparticles, respectively. Such a nanocomposite offers the possibility of separating a target efficiently from a complex matrix by simple application of an external magnetic force, followed by direct determination using SERS. High selectivity was achieved by the presence of anti-IgG on the surface of silver nanoparticles coupled with their further inactivation by ethylamine. Compared to many recently developed sandwich methods, application of single nanocomposites showed many advantages, including simplicity of use, direct control of the analytic process, and elimination of errors caused by possible nonspecific interactions. Moreover, incorporation of advanced spectral processing methods led to a considerable decrease in the relative error of determination to below 5%.