The 24-year-old, whose two-year spell with the Terriers ended in July 2011, has netted eight goals in just 13 starts for Derby this season.
Huddersfield boss Mark Robins is happy to secure the services of a proven goal scorer following the injury to Jermaine Beckford.
Robins said: "Theo gives us another dimension in our attacking options, particularly with Jermaine Beckford on the sidelines at the minute; his availability comes at a good time for us.
"He has a proven goal threat at Championship level, as you can see from his record. I've seen him play for Derby this season and he's played well for them, scoring a few when used as a starter and also off the bench. He has the ability to score a goal.
"Our supporters will be very familiar with him and know that he is pacey, direct and works hard. I'm looking forward to having him in the building tomorrow and working with him."