Estrogen Receptor-positive (ER+) breast cancer is the most common subtype of breast cancer. Despite standard-of-care endocrine therapy combined with CDK4/6 inhibitors (CDK4/6i), resistance is nearly universal in patients with ER+ metastatic breast cancer (MBC). Recent studies indicate that a subset of treatment-resistant ER+ MBCs harbor somatic alterations in RAS GTPases or the RAS-GAP NF1. However, no targeted therapies are approved for these patients. We analyzed a real-world next-generation sequencing dataset (Tempus Lens) of tumor tissue (n=2,302) and liquid (n=4,161) biopsies from stage 4 ER+ MBC obtained after endocrine therapy + CDK4/6i, likely representing resistant disease. Approximately 12% harbored somatic RAS pathway alterations, including pathogenic NF1 (6.4-7.9%) and KRAS (3.7-4.3%) mutations, with <1% affecting HRAS or NRAS. In contrast, only 3.1% and 1.6% of tissue biopsies from stage 1-3 ER+ disease (n=575) harbored NF1 or KRAS alterations, respectively, suggesting these alterations are acquired in late disease upon treatment pressure. We hypothesized that mutational activation of KRAS drives resistance to endocrine therapy and induces signaling vulnerabilities that can be exploited therapeutically. To test this, we generated CRISPR knock-in models of MCF7 and T47D ER+ breast cancer cells harboring KRAS G12V , the most common RAS mutation identified in our dataset. In both cell lines, KRAS G12V drove resistance to estrogen deprivation (mimicking aromatase inhibitor treatment) and total ER blockade (estrogen deprivation + Fulvestrant), while parental cell growth stalled. KRAS G12V also conferred resistance to CDK4/6i + estrogen deprivation. In basal and estrogen-deprived conditions, KRAS G12V upregulated MAPK and PI3K signaling. KRAS G12V expressing T47D cells also increased Ser118 ERalpha levels in full media, suggesting oncogene-ERalpha crosstalk. Treatment with the RAS(ON) multi-selective inhibitor daraxonrasib completely abrogated the estrogen-independent proliferation of KRAS G12V cells. To determine signaling dependencies downstream of KRAS G12V we treated cells with the mTORC1-selective inhibitor RMC-5552 (mTORC1i) or the ERK1/2 inhibitor ulixertinib (ERKi). Whereas treatment with mTORC1i potently inhibited KRAS G12V expressing cells and parental controls, ERKi partially reversed KRAS G12V antiestrogen resistance. In summary, we have demonstrated that KRAS G12V is sufficient to activate MAPK and PI3K signaling and drive resistance to standard-of-care ER+ breast cancer therapy, which is reversed by RAS inhibition. We are actively investigating the underlying mechanisms and applying CRISPR knockout screens and RNA-seq to identify other vulnerabilities. Given the recent surge of clinically viable RAS inhibitors, these findings may inform new combination strategies for a highly refractory subset of ER+ breast cancer.