Current cancer therapies block tumor survival and proliferation but fail to target metastatic dissemination and chemoresistance. Obscurin (720-870 kDa), a giant signaling protein that localizes to the breast epithelial cell membrane, is a metastasis suppressor and chemosensitizer commonly lost in breast cancer. Obscurin loss upregulates the Phosphoinositide 3-kinase/Protein Kinase B (PI3K/Akt) axis, which is altered in 30-40% of invasive breast carcinomas. The obscurin-pleckstrin homology (PH) domain interacts with the PI3K-p85 regulatory subunit. Here, we demonstrate that ectopic expression of membrane-targeted obscurin-PH domain in aggressive breast cancer cells, via adenoviral or lipid nanoparticle delivery, sequesters p85, suppressing PI3K/Akt activity. p85 sequestration eliminates filopodia, hampering migration and adhesion to pre-metastatic niche extracellular matrix substrates. This intervention also eradicates invadopodia, and reduces matrix metalloproteinase expression, blocking invasion, dissemination, and metastasis. We recapitulate this phenotype using the structurally homologous kalirin and phospholipase C gamma-1 (PLCγ1) PH-domains, and ultimately uncover, via in silico mutagenesis analysis coupled with stimulated emission depletion microscopy, a family of 9 PH-domains as p85-regulators, pinpointing the p85 inhibition metastasis suppressor (PIMS) motif that mediates this effect. In addition to tumor metastasis, our data demonstrate that the obscurin PH-domain synergizes with anthracyclines to enhance human epidermal growth factor receptor 2 positive (HER2+) breast cancer cell apoptosis. Furthermore, our findings reveal that breast epithelial cell obscurin loss primes the pulmonary vascular smooth muscle cell pre-metastatic microenvironment. Taken together, this work expands the pro-tumorigenic phenotypes driven by aberrant PI3K/Akt activity mediated by breast cancer OBSCN loss and solidifies the critical role the obscurin PH-domain plays in PI3K/Akt-mediated metastasis and chemoresistance. As PI3K inhibitors targeting the p110 catalytic subunit are limited to metastatic hormone receptor positive breast cancer, this work uncovers a new class of PI3K inhibitors in the form of the PH-domain, targeting the p85 regulatory subunit and setting the pace for the design of a novel therapy to combat metastatic breast cancer and chemoresistance.