The 35-year-old is in the twilight of an established career in which he has enjoyed spells at Fulham, Birmingham, Wigan and West Brom among others.
He was forced to retire back in October after being diagnosed with testicular cancer yet, after making a full recovery, he ended last season with Lincoln where he made 17 appearances.
But Adams has now enticed 'The Horse' into a management/coaching/playing role at Vale Park, and he cannot wait to get started.
Horsfield told the club's official website: "I am looking forward to it, it's something I've always wanted to do. From turning 30 I've always wanted to come into the coaching side and Micky Adams has given me the chance to keep on playing, which I could have done at a couple of clubs, but this is a chance to do my coaching and be an assistant manager.
"The problems I have encountered over the last 12 months are now put behind me. I'm going to learn a lot with Micky Adams. He has given me a leash to work with the strikers and do some scouting as well, but first and foremost I've got to get my fitness because I want to come in and play, not just come in and be assistant manager."