The Pro Bowl fullback signed a two-year contract Monday after being released by the Ravens in June.
"He's in shape and ready to go. And we're pleased to have him back," Harbaugh said. "Having Vonta back gives you a physical presence for sure with your offense, short yardage, goal line. Also, it's a leadership presence."
The Ravens released Leach last month when the sides couldn't come to terms on reducing his $3 million salary this year. But the Ravens reportedly had a standing offer of $2 million plus incentives and had been in contact with Leach since parting ways.