The 37-year-old Zaun is expected to replace Ramon Hernandez as the starting catcher, at least until the team's top prospect, Matt Wieters, is ready.
Zaun has 14 years of experience to draw upon in working with a young pitching staff and newcomer Koji Uehara of Japan. Zaun has a .251 batting average in 1,114 games with Baltimore, Florida, Texas, Kansas City, Houston, Colorado and Toronto.
His contract calls for a $1.5 million salary this year, and the Orioles have a $2 million option for 2010 with a $500,000 buyout.
To make room for Zaun on the roster, the Orioles designated first baseman Oscar Salazar for assignment.