Regular first team football at Premier League Albion has been limited for the 20-year-old so a move to Bramall Lane will be beneficial for himself and the Blades.
United have been leaking in goals left, right and centre this season, not to mention conceding 16 goals in the last five games alone!
Mattock's arrival will no doubt shore up United's troubled backline, and Blades boss Micky Adams seems pleased to have him here.
He said: "Joe's young and enthusiastic and certainly gives us competition for places. He had an injury at the start of the season which limited his opportunities for West Brom, but he's naturally left footed and having him here will help us climb the table."
Mattock goes straight into the squad ahead of this weekend's trip to Coventry City.
WBA do not have a recall clause in the loan agreement.
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