Ludwick, 33, hit .238 (90-for-378) with 18 doubles, 11 home runs and 64 RBI in 101 games with San Diego this season, making 95 appearances in left field and committing just one error in 167 total chances.
"Our scouts still saw the hitting abilities that made Ryan a successful hitter," Huntington said. "There's every reason to believe he's going to be a more productive hitter than his surface-level numbers indicate.
Now in his ninth big league season, the right-handed hitting Ludwick made his Major League debut with the Texas Rangers in 2002. He has also seen action with Cleveland (2003-05) and St. Louis (2007- 2010) before being traded to San Diego last year on July 31.
"This is a lot like last year," Ludwick said. "The only difference is, I've been hearing from day one, even in spring training, that I was a possible (trade) piece. It seemed every time I looked, teams were inquiring about me."
In 752 career games, Ludwick has compiled a .262 batting average, 115 homers and 439 RBI.
To make room on the 40-man roster, the Pirates have designated RHP Mike Crotta for assignment.
"Pittsburgh is a good fit for me," Ludwick said. "I've fared pretty well in (PNC Park) and I'm familiar with the Central. And I'm excited to be going to a team in a pennant race."