Nationality while not listed on koseki

Is is possible to obtain a Japanese passport under the following circumstances?

- One Japanese parent.
- born in the US
- born prior to 1985, so Japanese nationality should be implied.
- Not listed on family Koseki.

No, it's not possible right now.
You have to move to Japan and live here for at least 3 years with a Child of a Japanese visa. You can then apply for citizenship.

I believe that even if you were born to a Japanese parent, then back then your parents would have had to either (1) report you as the child to Japanese authorities, or (2) report that you are eligible to both American & Japanese nationalities and that you want to reserve the right to the Japanese nationality (while you also have another nationality), within 3 months of the birth, otherwise you lose Japanese nationality.

If you failed to do (2), if you are under 20 and reside in Japan, you may be able to re-gain Japanese nationality. But for the rest of it, you would have to follow the path of "Child of Japanese national," if your Japanese parent resides in Japan, and proceed to "Permanent Resident" then "Naturalization" if you want to obtain Japanese nationality.

FYI, PR is not a prerequisite for naturalization and can be skipped en route to citizenship.

if you are naturalized in Japan, you lose your US nationality.

ken's comment about losing U.S. citizenship is not true.

As I understand it, the Japanese government expects you to give up your U.S. citizenship when you naturalize, but the U.S. government does not regard such people as no longer being citizens.

U.S. citizens are subject to taxation on their worldwide income, so if it was easy to give up your citizenship, a certain number of wealthy people would do so in order to avoid U.S. taxes.