This international, multicenter, randomized, double-blinded, placebo-controlled study is designed to compare the efficacy and safety of taselisib + fulvestrant with that of placebo + fulvestrant in postmenopausal women with estrogen receptor (ER)-positive, human epidermal growth factor receptor-2 (HER2)-negative, oncogene that encodes for phosphatidylinositol-4,5-bisphosphate 3-kinase (PIK3CA)-mutant, unresectable, locally advanced or metastatic breast cancer after recurrence or progression during or after an aromatase inhibitor (AI) therapy. There will be a 2:1 randomization to the taselisib arm versus the placebo arm. Enrollment will be enriched for participants with PIK3CA mutant tumors via central testing. The anticipated duration of the study is approximately 3.5 years.