BEIJING—China launched a formal investigation into one of the Communist Party's most senior figures, lifting a cloak of immunity that has shielded the country's highest ranks as President Xi Jinping made his boldest attempt yet to solidify power.

Zhou Yongkang, China's former domestic security chief and one of the nation's top nine leaders until he retired in 2012, will be investigated for "serious disciplinary violations," according to the official Xinhua news agency, which didn't detail specific allegations against...