Overseas ace Siddle has been ruled out for the second consecutive season due to back problems but the Trent Bridge outfit have been able to land a high-class replacement.
Pattinson, whose brother Darren won the County Championship title with Nottinghamshire in 2008, has been plagued by his own injury issues recently, making the last of his 17 Test appearances for Australia against New Zealand in February 2016.
The 26-year-old, who has taken 70 Test wickets for the Baggy Greens at an average of 26.15, will be available for the County Championship and 50-over One-Day Cup until the end of June.
"As soon as James became available as a replacement, we knew he was a great option," said head coach Peter Moores.
"He's got real pace and he's very aggressive, which is the way we want to play our cricket. James has the ability to swing the ball, he hits good lengths and he does it all at a good speed.
"Pace creates opportunities so he gives us a wicket-taking option in both Championship and 50-over matches. We've seen that from him when he's played international cricket.
"That ability to open up sides on flat pitches is what we're really looking at. That's the kind of thing that wins you games."