The 27-year-old striker will link up with the fellow Championship club until May 31 2015.
Best joined Rovers from Newcastle United in the summer of 2012, but a cruciate knee ligament injury meant his first team debut didn't arrive until March 2013.
His first Rovers goal came in the 1-1 draw against the Rams on the opening day of last season, but after failing to hold down a regular place in the starting XI he joined Sheffield Wednesday for the second half of the season, scoring five goals in 16 appearances.
Best has been given permission to play for Derby in the Capital One Cup and FA Cup, but will be ineligible to play against Rovers when the two teams meet at Ewood on September 17th and in the return fixture on January 24th.
The Rams start their season at home to Rotherham United this Saturday.