The winger, 30, has joined the Pilgrims on a player/coach role, the club have confirmed.
Arnold leaves Lincoln City and Boston beat off competition from other clubs to get his signature.
United manager Craig Elliott says the former Salford and Grimsby man will bring a double boost as the club look to put up a fight for promotion in the National League North.
Elliott told the club’s website: “Persuading Nathan to join us is a real coup for the football club given the other offers he had, but his enthusiasm to join Boston United was evident.
“Nathan will play a key role on and off the pitch for us and I have offered him the opportunity to become an integral part of my management staff. We have had good discussions and I am looking forward to working closely with him.
“It is an exciting move all-round and kicks off our summer recruitment in style.”