United have completed the signing of Watford defender David Mirfin on a two-year deal.The centre-back signs after his contract at Vicarage Road was terminated by mutual consent.
The move sees Mirfin, who spent three years with United between 2008 and 2011, return to Glanford Park for a third spell.
The dependable defender made 111 appearances in all competitions for Scunthorpe in that period, scoring five goals and helping the North Lincolnshire side to promotion from League One via the play-offs in his first season.
The 27-year-old, who joined the Iron from Huddersfield Town, moved to Watford last summer after his contract expired, but he has played just five times in all competitions for the Championship outfit.
He returned on loan to United in the second half of last season, scoring once in 19 appearances and helping his team to safety.