Stephens, 28, becomes the Cherries' sixth signing of the window and joins after 11 years at Southampton.
Bournemouth are currently managerless after Scott Parking was sacked after their 9-0 defeat at Liverpool.
"We're very pleased to bring Jack to the club and he strengthens our defensive options," said Bournemouth chief executive Neill Blake.
"He brings with him significant Premier League experience and I'm sure will prove a very valuable addition both on and off the pitch."
More to follow.
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