The 27-year-old was part of the extended Black Caps squad in the United Kingdom to prepare for the three-game test series against England.
He was released when the squad was cut from 20 to 15 players, but has secured another playing gig.
The right-armer will be available for Kent in its next two County Championship matches against Gloucestershire and Surrey.
"It is a great opportunity to play some first-class cricket and I am looking forward to getting going with Kent," Duffy said in a statement.
Duffy had a tough summer for Otago in first-class cricket. He took 13 wickets at 54.83, which was well below his career output of 210 wickets at 33.37.
But he is a swing bowler and should go well in English conditions with the Dukes ball.
Kent director of cricket Paul Downton said the county was delighted to secure Duffy’s services.
"It is a very busy part of the season with Championship matches intermingled with the Vitality Blast. Jacob will strengthen our seam attack at this crucial stage of our Championship campaign.”
