Despite significant research regarding metastasis, there has been limited success in preventing it. However, gold nanoparticle (AuNP) technology has shown the potential to inhibit metastasis. Our earlier studies of gold nanorod-assisted plasmonic photothermal therapy (AuNRs-PPTT), where gold nanorods (AuNRs) were irradiated with near-infrared (NIR) light to induce heat, were utilized in slowing cancer cell migration in vitro. Herein, we have expanded the in vitro studies of the AuNRs-PPTT to xenograft mice to inhibit metastasis of mammary gland tumors. The study duration was 32 days from 4T1 cancer cell injections in four treatment groups: control (PBS), NIR Only, AuNRs, and AuNRs + NIR. Multiple AuNRs-PPTT treatment sessions with intratumoral AuNRs injections were conducted every 7 days on average on the mice. Photoacoustic spectroscopy has been utilized to study the distribution and aggregation of AuNRs within the tumors and the drainage of particles to the sentinel right subiliac lymph node. The photoacoustic results revealed that the AuNRs' shapes are still stable regardless of their heterogeneous distributions inside the mammalian tumor and lymph nodes. Bioluminescence imaging was used to monitor metastasis using luciferin labeling techniques and has shown that AuNRs-PPTT inhibited metastasis completely within the first 21 days. Moreover, proteomics was run to determine the most pivotal inhibitory pathways: NETosis, cell growth, cell proliferation, inflammation, and extracellular matrix (ECM) degradation. These five mechanisms are interdependent within related networks, which synergistically explains the molecular mechanism of metastasis inhibition by AuNRs-PPTT. The current in vivo data ensures the viability of PPTT applications in inhibiting metastasis in humans.