In July 2009, Treacy joined Sheffield United on a six-month loan deal, making his début for the club in a 0–0 draw with Middlesbrough on the first day of the 2009–2010 season. He soon followed his debut up with two assists for the Blades against Watford the following week and a stunning long range goal three days later in a midweek match against Leicester City. Since joining the club, Treacy started the season as a regular in the Blades side despite facing an injury that almost saw him sidelined for six months. By November, he had dropped out of the team and terminated his loan stay early to return to Blackburn in mid-December