The 30-year-old centre-back has been a free agent since he triggered a clause in his contact allowing him to leave Scottish Premiership side Hibernian in January.
Ambrose has spent the last week training at Derby, with manager Frank Lampard confirming to the Daily Record that he was considering offering the Nigeria international a contract, claiming that his commitment and ball-playing style would make him a good fit at Pride Park.
In quotes reported by Score Nigeria, the former Celtic defender has now claimed that a deal has been agreed with the Championship play-off hopefuls.
“I signed a short-term deal with Derby County that runs till the summer. I'm delighted to join this ambitious club,” said Ambrose, who has been handed the number 12 shirt.
“I am honoured and it is a privilege to play under a legend like Frank Lampard.
“The club’s target is to get the automatic promotion or playoff spot. I pray that I could be able to play a big part of that by helping them achieve this by God’s grace.”
Ambrose should bring experience and a winning mentality to Pride Park, having won four Scottish Premiership titles during his time at Celtic.
The Daily Record reported in January that a number of other Championship clubs - Sheffield Wednesday, Bolton Wanderers, Wigan Athletic and Derby’s East Midland neighbours Nottingham Forest - were also interested but Ambrose appears bound for Pride Park.