—Alteration of basalt is a ubiquitous process on the vast oceanic crust surface and results in the formation of secondary-phase minerals that include clay minerals and Fe-(oxyhydr)oxides. Thus, this process is a significant consequence of water/rock interactions that could reveal the (bio)geochemical conditions of formation. Core samples at the basalt/sediment interface from a depth of 74.79 m below sea floor (mbsf) were recovered during the International Ocean Discovery Program (IODP) expedition 329 (2010.10.10–2010.12.13) in the South Pacific Gyre (SPG). Two distinct regions of yellow- and red-colored sediment were observed. The mineralogy, elemental composition, Fe oxidation state, and mineral structure of the altered basalt samples were analyzed using transmission electron microscopy (TEM) with selected area electron diffraction (SAED) patterns, energy dispersive spectroscopy (EDS), electron energy loss spectroscopy (EELS), and micro X-ray fluorescence (µ-XRF). In the yellow sediment, K-nontronite and feroxyhyte (δ’-FeO(OH)) were the dominant mineral phases, while Mg-rich smectite (saponite), chlorite, and hematite were found predominantly in the reddish sediment. The appearance of K-nontronite and feroxyhyte mineral assemblages in altered sediment indicated that oxidative conditions prevailed during basalt alteration. Variation in the Fe-oxidation states in the K-nontronite structure, however, may indicate that local reducing conditions persisted throughout the biogeochemical reactions.