“He’s got a winning mindset, has played at the highest level and we’re delighted to get someone of his calibre to the club.”
The 30-year-old, who has also played five Tests and 22 one-day internationals, will make his Steelbacks debut in a home game against Lancashire on 5 July.
As currently scheduled, his final appearance will be away to Durham nine days later.
“There’s a really exciting mix of players in the squad and hopefully we’re in a really strong position in the table when I join,” Agar said.
Northants won the T20 Blast in 2013 and 2016 but last summer finished sixth in North Group, winning six of their 14 games.
They begin this year’s competition at home to Derbyshire on Thursday evening.