The deal was finalized following a physical and is for $2 million.
Adding another proven right-hander to the bullpen became a major need for Seattle after Casey Sadler was lost for the season when he required shoulder surgery.
Romo, 39, spent last season with Oakland, the sixth different team in his career. Romo had his greatest success during his nine seasons with San Francisco. Romo has also pitched for Tampa Bay, the Los Angeles Dodgers, Miami and Minnesota.
Last season with Oakland, Romo appeared in 66 games and had a 4.67 ERA and 60 strikeouts in 61⅔ innings.
"Sergio's stuff and style bring a unique look to our bullpen," Seattle general manager Jerry Dipoto said. "He is incredibly competitive and has shown the ability to rise up in big moments throughout his long major league career. He is a solid fit for us."
Romo has 798 career appearances and a career 3.10 ERA.