The 24-year-old winger was a regular in the England squad last season, but has made just seven appearances for Spurs in all competitions this campaign and has not started a Premier League game.
He has signed for an undisclosed fee, although British media reported the deal to be worth around 12 million pounds.
"As soon as I heard Newcastle were interested in me, there was only one place I wanted to go so I can't wait to get started," Townsend told the Newcastle website (www.nufc.co.uk)
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Townsend's future at Tottenham was thrown into doubt in November when he was suspended for two weeks after clashing with fitness coach Nathan Gardiner. The winger has been training with Tottenham's under-21s since then.
Townsend becomes Newcastle's third signing of the transfer window after midfielders Henri Saivet and Jonjo Shelvey.
"Bringing Andros to the club is a great signing," Newcastle manager Steve McClaren said.
"He is a very good talent who is a current England international and should be one for years to come too."
(Reporting by Toby Davis; editing by Ed Osmond)
