OBJECTIVE: To develop a novel Magnetic Resonance Imaging (MRI)-guided Near-Infrared Spectroscopic Tomography (MRg-NIRST) imaging system with an MRI-compatible breast optical interface for breast imaging. METHODS: The breast interface consists of eight flexible opto-electronic circuit strips, each equipped with six photodetectors and six side-firing fiber-probes. Concurrent MRI and NIRST data were acquired from a total of 2,304 source-detector positions at six wavelengths, enabling 3D MRg-NIRST image reconstruction of the entire breast. The system was validated through a series of phantom and normal subject studies. RESULTS: Reconstructed images of phantoms with inclusions ranging 10-25 mm in diameter showed errors in the estimated inclusion diameter and contrast of total hemoglobin (HbT) within the inclusion relative to the background were ranged in [-7%, 12%] and [7%, 28%], respectively. HbT estimates from reconstructed images of nine normal subjects ranged between 8.0-l25.2 μM, align with previous imaging studies. CONCLUSION: Results from both phantom and normal subject studies indicate that this system has the potential to be easily integrated into clinical practice for acquiring 3D MRg-NIRST images of the entire breast. SIGNIFICANCE: The flexibility of the wearable breast optical interface, along with the increased number of sources and detectors, has improved the optical accessibility for breasts of various sizes, shapes, and tumor locations. 3D MRg-NIRST image reconstruction, based on optical data collected from multiple source-detector layers across the entire breast, demonstrates that MRg-NIRST is ready to be tested clinically for its potential to enhance breast cancer detection alongside MRI.