The 30-year-old will be available to Mick Newell's side until June 21, with his stay covering seven County Championship games and eight one-day fixtures.
Cowan, who has become a fixture at the top of Australia's batting line-up over the last 12 months, will be able to get used to the wicket at Trent Bridge, which hosts the opening Ashes Test on July 10.
"I was very keen to play some division one cricket and I`m pleased to have the opportunity to do that with Nottinghamshire," the New South Welshman said. "There are a lot of good players at the club and Trent Bridge is a great venue so I`m hoping to make a good contribution on and off the field."
Cowan averages 34 across 11 Tests, with a 136 against South Africa last month his best score to date.