The 'lens' itself is...but it has to have the right mount. You probably just have a version of the lens with a different mount. Take it back and get one that has the 'Canon EF' mount.
The Canon 350D is part of the Canon EOS system...and it can use any of the Canon EF or EF-S lenses...or any other lens (Sigma, Tamron, Tokina, Vivitar etc.) that has the EF mount.
Big Mike I thank you, now we're cooking.
At least I know roughly what I am on about when I go back in and flame his pre-pubescent ass!