The 6ft 5in centre back will join the club in the summer on a two-year deal, following the expiry of his current contract with Livingston.
“We have been a long-term admirer of Declan,” Motherwell boss Stephen Robinson said of the deal.
“We are very pleased to have confirmed him for next season.”
Gallagher, 28, came through the youth ranks at Celtic but kicked off his career with a loan spell at Stranraer, before moving to Clyde.
His performances for the Bully Wee then persuaded Dundee to buy him for an undisclosed fee back in 2012.
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He remained at Dens Park for two years before joining Livingston, where he has been ever since, amassing over 150 appearances and helping to the club to back-to-back promotions in the 2016/17 and 2017/18 seasons.
Livi have impressed in their return to the top flight, with Gallagher in particular being a standout.
Statistically, he tops the entire league for aerial duels contested and he is second only to Aberdeen’s Graeme Shinnie for defensive duels competed in.
