The 23-year-old right-back left Preston, where he was managed by current Rams boss Billy Davies, for West Ham in a �1m deal last summer.

But he has made just three Premiership starts, plus two substitute appearances after moving to Upton Park.

"He is a very attacking full-back with excellent pace who adds great competition," said boss Davies.

"Ideally we would like to make this deal permanent in the summer."

Mears becomes Derby's seventh signing of the transfer window as the Championship leaders push for a place in the Premiership.