The combative midfielder has turned out seven times for County this season and will look to become more of a first-team regular at Valley Parade.
Bantams boss Phil Parkinson will be hoping Ravenhill, 30, can be the catalyst who propels the club up the League Two table from their lowly position of 21st.
“We’re pleased that Ricky has agreed to join us,” Parkinson told Bradford’s official website. “We’ve been looking for another midfield player since Adam Reed went back (to Sunderland).
“We feel it is important the squad is strong coming up to a busy period of games. Ricky will give us another option in that midfield.”
Ravenhill is eligible to make his debut against Rotherham on Saturday.
Watford boss Javi Gracia does not think Abdoulaye Doucoure's form will be affected by his omission from France's latest squad.