The 26-year-old requested a move to play regular first-team football.
The Wales international has made 13 Premier League appearances this season, scored one goal and had two assists in last week's FA Cup fourth-round romp against Swansea City.
"I want to be playing regular minutes and this opportunity was one I couldn't really turn down," said Brooks.
Brooks told the Saints website: "I'm really happy. With the past couple of years with my situation, just to come here and try and get some game time, I'm really looking forward to it and can't wait to get started."
Bournemouth chief executive Neill Blake told the Cherries website: "Having discussed David's situation with him this month, and taking into account his desire to be playing more minutes of first-team football, we have accepted his request to authorise such a move.
"There has been a certain level of reluctance on our part, as David means a great deal to us all at the club. It is because of this that we respect his wishes, and look forward to his return where he will continue to be a valuable asset to our squad, hopefully for many years to come."
Brooks joins Cherries team-mate Joe Rothwell in moving to St Mary's on loan this month, while former team-mate Ryan Fraser is also at Saints for the season on loan from Newcastle.
Find all the latest football transfers on our dedicated page.
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