Abstract Fulvestrant is used to treat patients with hormone receptor positive advanced breast cancer but acquired resistance is poorly understood. PlasmaMATCH Cohort A (NCT03182634) investigated the activity of fulvestrant in patients with activating ESR1 mutations in circulating tumor DNA (ctDNA). Baseline ESR1 mutations Y537S associated with poor, and Y537C with good outcome. Sequencing of baseline and EOT ctDNA samples (n=69) revealed 3/69 (4%) patients acquired novel ESR1 F404 mutations (F404L, F404I, F404V), in cis with activating mutations. In silico modelling revealed that ESR1 F404 contributes to fulvestrant binding to ERa through a pi-stacking bond, with mutations disrupting this bond. In vitro analysis demonstrated that single F404L, E380Q, and D538G models were less sensitive to fulvestrant, while compound mutations D538G+F404L and E380Q+F404L were resistant. Several oral ERa degraders were active against compound mutant models. We have identified a resistance mechanism specific to fulvestrant, that can be targeted by treatments in clinical development.