San Diego's chief soccer officer Tyler Heaps said: "Paddy is another addition to our Club that will be invaluable as we prepare for our inaugural season in MLS. He is a center-back that is extremely comfortable in possession and has the versatility to play a number of roles within our team.
"His wealth of experience and leadership as the captain of Northern Ireland will be a tremendous asset to our roster and locker room. We are excited to welcome him to San Diego and have him join our Club in January."