We conducted intensive atmospheric observations and surface seawater sampling in the western tropical Pacific aboard R/V Mirai during the MR21-03 cruise from May to July 2021 to investigate iodine variations and photochemistry over the global sea surface temperature (SST) maxima (warm pool). Consistent with previous studies, high near-surface atmospheric iodine monoxide (IO) mixing ratios of up to ~ 1 pptv were observed. A strong positive correlation (r = 0.95) was found between the IO mixing ratios and inorganic iodine flux calculated from atmospheric ozone (O3) and sea surface I– data, indicating that the assumed emission flux and the associated processes are qualitatively reasonable for the studied warm pool region. This study provided the first simultaneous observations of atmospheric O3, IO, and sea surface I– over the warm pool, thus demonstrating the robust nature of the relationship. The CHASER chemical transport model satisfactorily reproduced the observed positive correlation. The model overestimated the IO levels by 40%, but large systematic uncertainties were associated; refining the assumed emission flux and parameters used in the model is recommended for more quantitative comparisons. The collective findings underscore the significance of concurrent observations of atmospheric O3, IO, and sea surface iodide for elucidating ocean–atmosphere processes involving reactive iodine compounds.