The 27-year-old has had enough experience at this level and will vie for a place in the Blades' starting 11 in next terms' promotion hunt. Williamson turns out to be United's first recruit of the close season, with several more expected to come through the door.
Willimson passed a medical this afternoon (Thursday) and joins a couple of ex Hornets. Darius Henderson and Leigh Bromby have both had spells at Vicarage Road respectively.
Williamson, nicknamed 'Willio', spent the back end of last season on loan at Preston North End and has signed a three-year-contract at BDTBL.
Meanwhile Kyel Reid is expected to join later today or in the next couple of days according to a number of media organisations.