The 26-year-old former frontman's move to St James' Park was rubber-stamped, the club have confirmed.
Ba, who scored seven times in 13 appearances for the Hammers during the second half of last season, came 24 hours after Magpies skipper Kevin Nolan headed for Upton Park in a deal believed to be worth in the region of ?4million.
Former Hammers manager Alan Pardew said: "Demba made a great impression with West Ham last season and is a great signing for this club.
"He has pace, excellent technical ability, is good in the air and, it goes without saying, has the ability to consistently find the back of the net.
"Moreover he has a great passion to play for Newcastle United."
Newcastle beat off competition from a series off other clubs to land the former Hoffenheim player as Pardew looks to increase his options in attack.
Despite his best efforts we suffered relegation last season, but not before he had proved both his fitness and his prowess in front of goal.