Certificate of Finnish nationality

[FinnGuyHelsinki]

A passport? Didn't the party that is requiring the certificate specify what is accepted? If you haven't asked them, I'd suggest to do that. If you have applied for a Finnish citizenship and have had it granted, Migri should've sent you the decision.

[interleukin]

I guess it could also be an extract from the population registry, with the nationality written somewhere?

[Rip]

I guess it could also be an extract from the population registry, with the nationality written somewhere?

It seems to be there by default. I'd try (with obvious modification) what is recommended here for getting the (here really non-existing) birth certificate: viewtopic.php?f=3&t=81184#p514008

(So get population registry extract, give as reason "proof of nationality" and get it apostilled or legalized)