In the past two years, Agar has taken 41 wickets in 11 first-class matches. He was named the Bradman Young Cricketer of the Year in 2020, awarded to Australia's most promising first-class player under the age of 24.
Agar will be available to play until May 21, covering home matches against Essex and Hampshire and away fixtures with Middlesex at Lord's and Surrey at The Kia Oval before Arshdeep arrives in June.
"This is an exciting new challenge for me," Agar said. "Playing county cricket has always been a dream of mine and I'm excited to be a Kent player and thankful for the opportunity. With a win already under our belt this year, I hope to be a part of more victories in my time here."
Kent defeated Northamptonshire in their opening match of the season last week while their ongoing Division One clash at Warwickshire was one of several Championship games delayed by rain on the second morning on Friday, with the hosts 367 for 3 overnight after being sent in to bat and punishing a depleted attack missing injured seamers Nathan Gilchrist and Grant Stewart. Matt Quinn, the New Zealand quick also broke down early in his new-ball spell before returning to action after the lunch break.
Paul Downton, Kent's director of cricket, said: "Wes is an exciting, wicket-taking bowler known for bowling a 'heavy ball'. He offers us a point of difference that will complement our core bowling group at the club and will help us in our efforts to take 20 wickets in each of our next four matches in the Championship."
Kent recently announced that they had signed Arshdeep for five Championship matches between June 11 and July 28. He and Agar join Kane Richardson, the Australia pace bowler who will play in the Vitality Blast, and South African spin-bowling allrounder George Linde, who is in the second half of his two-year all-format deal, as overseas players at the club.