The 33-year-old second row, who has 23 Scottish caps to his name, will join the club for the 2012/13 season, whichever league the Falcons are playing in.
Newcastle, who finished bottom of the Aviva Premiership, are waiting to see whether London Welsh win the Championship and therefore earn a place in the top flight. If Cornish Pirates, whose ground is not up to scratch, beat Welsh in the two-legged final later this month, Newcastle will survive.
Hawick-born MacLeod joins the Falcons on the back of a year's stint with Japanese outfit Kobe Steelers. He has also played for Border Reivers, Scarlets and Edinburgh.
MacLeod told the club's official website: "Newcastle Falcons are a club with great potential and I am really looking forward joining up with my new team-mates.
"I am good mates with Ally Hogg and he has told me how good the team spirit is at the club, and it can only go from strength to strength under the management of Dean Richards."
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The entire All Blacks apparel line has been updated for 2013/14. Check out the New Zealand All Blacks polo.
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