The 29-year old joins from Danish club FC Midtjylland and is looking forward to a fresh challenge with a brand new team.
"At this point in my career - being a veteran - I'm quite comfortable being asked to take on a leadership role," the California native said. "I am so excited to work with American kids, who, like me, have come through the U.S. national team system. I'm sure we'll relate well to one another for the benefit of Philadelphia Union."
Phillie coach Peter Nowak said the signing of Califf was an extremely important one for the fledgling club and expected him to bring a wealth of experience to the youth at the Union.
"With a seasoned veteran like Danny Califf, we're adding a player who will help us set the bar ever higher," Nowak said. "Danny understands the level of commitment our technical staff will demand daily, and we're counting on him to not only help anchor our defense, but push our younger players to greater heights, too."