A Combination Study of Rucaparib and Atezolizumab in Participants With Advanced Gynecologic Cancers and Triple-Negative Breast Cancer

Inclusion Criteria: * Eastern Cooperative Oncology Group (ECOG) Performance Status of 0 or 1 * A life expectancy of at least 3 months * Have disease that is measurable as according to RECIST v1.1 * Have sufficient archival formalin-fixed paraffin-embedded (FFPE) tumor tissue available for planned analyses * For Part 1, have a histologically confirmed diagnosis of ovarian or endometrial cancer, and have received at least one line of prior therapy for metastatic disease * For Part 2 ONLY, have disease that can be safely biopsied * For Part 2 ONLY, have a deleterious germline or somatic breast cancer susceptibility gene 1 (BRCA1) or BRCA2 mutation or tumors that are wild-type BRCA but show high levels of loss of heterozygosity (LOH) (tBRCAwt/LOHhigh) signature * For Part 2 Cohort 1 (ovarian cancer), high-grade serous or Grade 3 endometrioid epithelial ovarian, fallopian tube, or primary peritoneal cancer (PPC) * For Part 2 Cohort 1, have received at least one and no more than two lines of prior platinum-containing therapy and progressed after the most recent platinum therapy in a platinum-sensitive timeframe * For Part 2 ONLY, Cohort 1, have a CA125 measurement that is greater than 2 times the upper limit of normal (ULN) * For Part 2 Cohort 2 (TNBC), metastatic, histologically confirmed estrogen receptor (ER)-negative, progesterone receptor-negative, and HER2-negative adenocarcinoma of the breast per local laboratory assessment * For Part 2 Cohort 2, radiologic/objective evidence of recurrence or disease progression after one line of chemotherapy for TNBC in the metastatic setting * Have adequate organ function Exclusion Criteria: * History of prior malignancy except a) curatively treated non-melanoma skin cancer, b) solid tumor treated curatively more than 3 years ago without evidence of recurrence, c) For Cohort 1 (ovarian cancer): breast cancer with no evidence of disease or inactive for at least 3 years, and d) synchronous endometrial cancer (Stage 1A) with ovarian cancer * Treatment with chemotherapy, radiation, hormones (except corticosteroids and megestrol acetate), or other anticancer therapies less than or equal to (\<=) 14 days prior to first dose of study treatment * Preexisting duodenal stent and/or any gastrointestinal disorder or defect that would, in the opinion of the investigator, interfere with absorption of rucaparib * Symptomatic and/or untreated central nervous system metastases * Prior treatment with any poly adenosine diphosphate-ribose polymerase (PARP) inhibitor