Do Japanese manga magazines have ISSNs? I know Viz's Shonen Jump has an ISSN (1545-7818), but that's from North America.

Do any Japanese manga magazines (e.g. Weekly Shonen Jump, Monthly Shonen Gangan, Dengeki Daioh, Comic Yuri Hime, etc.) have ISSNs?
By all indications, no.

There are a few places I checked. First, while Wikipedia gives the ISSN for magazines such as Shonen Jump but doesn't give it for magazines such as Weekly Shonen Jump or LaLa.

Second, I looked at OCLC WorldCat, which similarly gives the ISSN for Shonen Jump but not Weekly Shonen Jump or LaLa. (Those are just some random popular magazines I picked, I checked for others as well, I just don't want to list them all.)

Lastly, I checked the ISSN International Centre, which has Shonen Jump but can't find Weekly Shonen Jump or LaLa. The reason I didn't mention this first is that a lack of search results is not always the best indicator of something not existing, particularly when there are issues across languages, but I did search in both English and Japanese, so I'm fairly certain this means there just aren't ISSNs for them.

Extrapolating out, it can be assumed that Japanese manga magazines don't have ISSNs. However, they do have 雑誌コード (magazine codes) that are used as identification codes for magazines/journals in Japan. The Wikipedia article is in Japanese but if you translate it, it provides a decent explanation. You can also see the magazine code (09206-06) to the left of the barcode in the magazine below.