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The Nazca Ridge (NR) was formed near the interaction of a hotspot mantle plume and an active spreading center. We use active‐source wide‐angle seismic data to obtain 2‐D Vp and Vs tomographic models, and hence the Poisson's ratio (ν) structure beneath the NR. Results show a ∼2 km thick seismic layer 2A with ν values of 0.25–0.32 in the uppermost crust interpreted as pillow basalts with a low degree of fracturing and/or hydrothermal alteration. The 2A/B boundary layer presents ν values of 0.27–0.29 consistent with pillow basalts/sheeted dykes units. A ∼3 km layer 2B overlies a ∼10 km layer 3 with ν values of 0.24–0.3 at the 2/3 boundary layer. The lowermost layer 3 presents ν values of 0.28 ± 0.02 suggesting an increase in Mg content (≥10% wt). The NR crust (∼15 km thick) requires an increment of the asthenospheric mantle potential temperature in ∼100°C formed by passive adiabatic decompression melting. Plain Language Summary: The interaction of a hot mantle plume with an overlying moving oceanic plate results in hotspot volcanic activity, and the formation of large volcanoes (hotspot tracks). This process is enhanced in a setting where the hotspot mantle plume is located near an active spreading center or mid ocean ridge such as the East Pacific Rise or Middle Atlantic Ridge. The Nazca Ridge (NR) is a hotspot track located off South America at present and was formed 30–45 Ma by the interaction of the Easter Island hotspot mantle plume with the Farallon‐Nazca plate near the East Pacific Rise. Our geophysical results show that the NR presents an anomalously thick oceanic crust (∼15 km thick), which is about 2.5 times thicker than normal crust of the Pacific Ocean. The rocks at the base of the crust presents likely an increase in magnesium content. This is consistent with the intrusion of voluminous magmatic material during the interaction of the hotspot plume with the oceanic plate at anomalously high temperatures compared with the surrounding mantle. Key Points: The seismic P‐ and S‐wave structures from wide‐angle seismic data yield a Poisson's ratio model beneath the Nazca Ridge (NR) prior to subductionThe NR presents a lower crust with Poisson's ratios of 0.28 ± 0.02 consistent with a high concentration of MgThe thick NR crust (∼15 km) is consistent with an elevation of the asthenospheric mantle potential temperature in ∼100°C [ABSTRACT FROM AUTHOR]