in or on

[allthewayanime]

What's the difference if I say:

I have seen it on a magazine.
I have seen it in a magazine.

[emsr2d2]

What's the difference if I say:

I have seen it on a magazine.
I have seen it in a magazine.

If you are talking about an article which was held inside a magazine (so you had to open the magazine in order to read it), then you use "in".

If you are talking about the photograph or some words on a magazine front cover, you might say "on" but you would be more likely to specify "I have seen it on the cover of a magazine".

If you are talking about another object which was sitting on top of a magazine, you might say "I have seen a coffee cup on a magazine".

Note that without context it's hard to tell whether "I have seen" is correct here. "I saw it in a magazine" sounds much more likely to me.

[shannico]

You are talking about some sort of content, even if it's only a picture, of an article or a classified section of a magazine. Therefore it should be IN a magazine.
I read it in a magazine.
I read it in a book.

However, you can say
It was on the cover of a magazine.
It was on the front page of a magazine.

Hope it helps