This open-label, multicenter, phase IIIb study will assess the safety, tolerability and efficacy of a combination therapy of intravenous (IV) pertuzumab (Perjeta), subcutaneous (SC) trastuzumab (Herceptin), and taxane chemotherapy (docetaxel, paclitaxel or nab-paclitaxel) as first-line therapy in participants with HER2-positive metastatic breast cancer (mBC). All participants will be treated with 3-week cycles of pertuzumab IV (840 milligrams [mg] first dose; subsequent doses of 420 mg) and trastuzumab SC (600 milligrams per 5 milliliter [mg/5 mL]). The taxane treatment regimen will be determined by the investigator. Participants will continue therapy until disease progression, unacceptable toxicity, or the participant withdraws consent, whichever occurs first. Time on study treatment is up to 2 years.