The 27-year-old centre-back, who was a free agent after leaving the Blades in the summer, has spent the past two season on loan with Blackpool.
Dons boss Glyn Hodges told the club website:
"He's no-nonsense, a good size, and he's a good professional.
"Before the deadline we had targeted a centre-back, but did not quite get that done. It's unusual to sign a player of Ben's calibre outside the window."