Chief executive Julian Winter told the club's official website: "First and foremost all at SUFC, from the board right the way through the club, wish David and Lee all the best for the future.
"None of us wanted to be in this position at this point in the season. David's appointment was made as part of developing a new phase at the club."
The club's under-21 coach and former Blades defender Chris Morgan, goalkeeping coach Darren Ward and head of academy coaching Travis Binnion will take charge of the team on an interim basis - starting with Sunday's televised trip to Sixfields to face Coventry.
The Blades insist they aim to appoint a new permanet management team as quickly as possible.
Winter added: "Chris Morgan has agreed to take an interim role in charge of Sunday's team. He knows the players and, with Darren and Travis, will be doing his utmost to get a return to winning ways."
Morgan added: "The club have made me aware of the situation and I am happy to step in and work with the players for the Coventry game."
Weir replaced Danny Wilson in the summer but endured a baptism of fire in his first management role, losing nine of United's 13 matches in all competitions.
Saudi Arabian Prince Abdullah bin Mosaad purchased 50 per cent of the club's share from owner and chairman Kevin McCabe last month and gave Weir his full backing.
But the club's fans have consistently expressed their concern at such a poor start to their third season in the third tier, which has seen the Blades lose five of their last six games.
Sheffield United have confirmed that they've sacked manager David Weir after only 13 games in charge.
A statement on the club's website says: "Sheffield United today announced that it has relieved first team manager David Weir of his duties along with his assistant Lee Carsley."
They've also confirmed that under 21 coach Chris Morgan - who also took temporary charge when Danny Wilson was sacked towards the end of last season - will again be overseeing the team on an interim basis. His first task will be to prepare them for the televised game at Coventry on Sunday.
The Blades are without a win in the League since the opening day of the season, and are currently joint bottom of League One.
Sheffield United have sacked manager David Weir. The Blades have not won in the League since the opening day of the season; they are currently joint bottom of League One; and in midweek they were dumped out of the Johnstone's Paint Trophy by a side near the bottom of League Two.
Weir - who turned up as normal to this afternoon's regular news conference - only took over in June. It was his first managerial appointment, but Tuesday's defeat at home to Hartlepool was the final straw. Former club captain Chris Morgan will be in caretaker charge for Sunday's game at Coventry.