The youngster already has League Two experience after previous loan spells at both Hereford and Northampton.
Threlfall, Peter Taylor's first signing as Bradford's new manager, goes straight into City's squad for Tuesday's night trip to Rochdale.
Liverpool-born Threlfall, who has had loan spells at Hereford in both the last two campaigns, made four appearances at Northampton earlier this season.
And he also had a loan spell with Stockport County last season.
Source: BBC Sport
Source: BBC Sport