We use time-dependent density functional theory and a semiclassical model to study second-harmonic generation in a system comprising a quantum emitter and a spherical metallic nanoparticle, where the transition frequency of the quantum emitter is set to be resonant with the second harmonic of the incident frequency. The quantum emitter is shown to enable strong second-harmonic generation, which is otherwise forbidden because of symmetry constraints. The time-dependent density functional theory calculations allow one to identify the main mechanism driving this nonlinear effect, where the quantum emitter plays the role of an optical resonator that experiences the nonlinear near fields generated by the metallic nanoantenna located nearby. The influence of the intrinsic properties of the quantum emitter and the nanoantenna, together with the relative position of both in the coupled system, allows for a high degree of control of the nonlinear light emission. The main effects and contributions to this nonlinear process can be correctly captured by a semiclassical description developed in this work. Keywords second-harmonic generation; plasmonics; quantum emitter; nonlinear optical response; electromagnetic coupling The coupling between incident electromagnetic radiation and collective electronic excitations, so-called surface plasmons, in metallic nanoparticles (MNPs) allows one to localize, enhance, and control the near fields around the nanoparticles at scales well below the wavelength of light. The resonant excitation of the plasmonic modes, along with the intrinsic nonlinearity of the metals, results in a strong nonlinear optical response of plasmonic structures with a great variety of applications. 1,2 In particular, second-harmonic generation (SHG), whereby two photons at the fundamental frequency are absorbed to emit one photon at the second-harmonic frequency is at the focus of very active research owing to its practical and fundamental interest. 3-12 For typical plane-wave incidence, the SHG is forbidden for materials and nanostructures that are centrosymmetric. This nonlinear response is thus very sensitive to the geometry of the system and to surface effects that eventually may break the symmetry constraints and lead to the emission of light at the second harmonic. 2,13-18 In this context, it has been shown that plasmonic structures resonant at the fundamental or at the second-harmonic frequency (or at both frequencies) can give rise to manyfold enhancement of the SHG. 8,9,19-31 Recent experiments have also shown the polarization-resolved probing of the nonlinear near field distribution of metallic structures by using doubly resonant plasmonic antennas. 32 On the other hand, the coupling of a quantum emitter (QE), such as an organic molecule or a quantum dot with a plasmonic nanoan-tenna has been widely studied in previous works, analyzing diverse aspects such as surface-enhanced Raman scattering, single-molecule spectroscopy, strong coupling or the effect of electronic conductivity through molecules. 33-41 Moreover, the capability of the MNP-QE interaction to modify the second-harmonic emis-1 sion has been demonstrated for plasmonic nanostruc-tures, 42,43 and also the strong nonlinear response of graphene nanostructures has been proposed as a way to excite the electronic transitions in atomic or molecular species. 44 Here we study the SHG resulting from a hybrid system consisting of a QE placed in the vicinity of a spherical MNP, as scketched in Figure 1a. The small individual centrosymmetric nanoparticle does not allow for second-harmonic emission, but the presence of the QE lifts this symmetry constraint. When the electronic transition frequency of the QE is resonant with the second harmonic of the incident frequency, the QE plays the role of an optical resonator, which efficiently couples to the nonlinear near fields induced around the nanoparticle, extracting them to the far field and thus producing SHG. 32 This MNP-QE system thus enhances the frequency conversion and allows for its control. To calculate the nonlinear response of the coupled system and to reveal the physical mechanisms behind the SHG in this situation, we use a quantum approach based on the time-dependent density functional theory (TDDFT). 45,46 With the insights obtained from the TDDFT calculation, we develop a semiclassical model. We show that, for the cases where the quantum calculations are doable, the semiclassical model reproduces the TDDFT results. This semiclassical model also allows for addressing more general and complex situations beyond the reach of TDDFT, so that it makes possible a detailed study of the sensitivity of SHG to different parameters that characterize the system. In particular, we demonstrate the polarization conversion of the non-linear signal, as well as the existence of various regimes of SHG determined by the intrinsic losses of the QE. The methodology and results obtained in our study can pave the conceptual road for enhancing and optimizing second-harmonic generation mediated by quantum emitters coupled to plasmonic systems. 47,48 TDDFT Calculations Nonlinear Response of an Individual Metallic Nanoantenna Prior to the discussion of the MNP-QE system, we analyze the nonlinear optical response of an individual spherical MNP calculated within TDDFT. We describe the electronic structure of the MNP within the Jellium model, which is well adapted to TDDFT studies and correctly addresses the quantum many-body dynamics of conduction electrons including the response to strong optical fields. 49-53 The ions at the metal lattice sites are represented by a homogeneous positive background charge of density n + = 4 3 πr 3