We explore the potential of the adjoint-state tsunami inversion method for rapid and accurate near-field tsunami source characterization using S-net, an array of ocean bottom pressure gauges. Compared to earthquake-based methods, this method can obtain more accurate predictions for the initial water elevation of the tsunami source, including potential secondary sources, leading to accurate water height and wave run-up predictions. Unlike finite-fault tsunami source inversions, the adjoint method achieves high-resolution results without requiring densely gridded Green’s functions, reducing computation time. However, optimal results require a dense instrument network with sufficient azimuthal coverage. S-net meets these requirements and reduces data collection time, facilitating the inversion and timely issuance of tsunami warnings. Since the method has not yet been applied to dense, near-field data, we test it on synthetic waveforms of the 2011 Mw 9.0 Tohoku earthquake and tsunami, including triggered secondary sources. The results indicate that with a static source model without noise, using the first 5 minutes of the waveforms yields a favorable performance with an average accuracy score of 93%, and the largest error of predicted wave amplitudes ranges between -5.6 to 1.9 meters. Using the first 20 mins, secondary sources were clearly resolved. We also demonstrate the method’s applicability using S-net recordings of the 2016 Mw 6.9 Fukushima earthquake. The findings suggest that lower-magnitude events require a longer waveform duration for accurate adjoint inversion. Moreover, the estimated stress drop obtained from inverting our obtained tsunami source, assuming uniform slip, aligns with estimations from recent studies.