The Raiders said Asomugha will announce his retirement officially at a news conference at team headquarters Friday.
Asomugha had his greatest success in the NFL with the Raiders after being drafted in the first round out of California in 2003. He earned three Pro Bowl selections in eight seasons in Oakland and was a first-team All-Pro in 2008 and 2010.
Asomugha struggled after joining Philadelphia as a high-profile free agent in 2011. After two seasons with the Eagles, Asomugha played three games with San Francisco this season before being cut last month.