The dynamic all-rounder has been snapped up to play for the full duration of the Blast, and is also available to play in Northamptonshire's LV= County Championship clash against Warwickshire at the end of June.
The move is subject to international clearance, but the 31-year-old is an exciting signing as the Steelbacks have landed a proven white-ball performer on the world stage.
Neesham was a central part of the New Zealand team that was narrowly beaten by England in the 50-over World Cup Final at Lord's in 2019, and was also in the Black Caps team beaten by Australia in the ICC World T20 Final in November.
In that thrilling 50-over final at Lord's, Neesham claimed three wickets and produced the final-ball run out to tie the scores at the end.
He was then sent out to bat in the Super Over and scored 13 from five balls, including a huge six off Jofra Archer.
But, crucially, he was was left at the non-striker's end for the final ball when Martin Guptill was run out coming back for a second run that would have won New Zealand the title.
In the ICC World T20 before Christmas, he played a major role in the Kiwis reaching that final, hammering a match-changing 27 from just 11 balls as Eoin Morgan's England team were beaten in the semi-final.
In all, Neesham has made 38 T20 international appearances for his country, scoring 416 runs at a strike-rate of just shy of 152 with a top score of 48 not out.
He has also taken 21 T20i wickets at 27.9 apiece, with best figures of three for 16.
Neesham has also played in 178 T20 games for various clubs, franchises and districts around the globe, scoring 2,338 runs at 22.92 with a strike-rate of 137.69.
He his managed five half-centuries in T20s, with a highest score of 59 not out.
With the ball he has claimed 155 wickets at 25.40, with a best haul of four for 24.
Last summer he turned out for Essex Eagles in the Blast, and he has also played for Derbyshire in 2016 and Kent Spitfires in 2017.
His other clubs in T20 cricket include Delhi Daredevils, Kings XI Punjab and Mumbai Indians in the IPL, as well as Guyana Amazon Warriers in the Caribean PL and Edmonton Royals in the 2019 Global T20 Canada tournament.
In his native New Zealand, he plays for Wellington.
Neesham was also a Welsh Fire team-mate of Steelbacks skipper Josh Cobb in The Hundred last summer.
He will be available for the Championship clash against Warwickshire due to the fact that the club's four-day overseas signing, Neesham's fellow Kiwi Will Young, is expected to be playing for his country in the three-match Test series against England.