HIF, a health-insurance fund based in Perth, will allow its customers to list their gender as "other" on forms.
The not-for-profit fund will also offer the honorific "Mx" on its forms as well as traditional titles.
Australian citizens can legally identify their gender on their passports as M, F or X.
This year's national census was also the first to offer a third gender category.
Grame Gibson, HIF managing director, said the decision to change the forms was made following feedback from the LGBTI community.
"HIF members may select the 'other' gender option if they choose to identify as neither male nor female, including (but not limited to) those who are agender, androgyne, bi-gender, gender fluid, intersex, neutrois, non-binary, pangender or transgender," he said in a statement.
The "Mx" honorific - pronounced "mux" or "mix" - is used to avoid specifying gender or marital status.
It already appears on some government forms in Britain and is used by many banks.
An insurance company has become the first in Australia to formally recognise customers who do not identify as male or female.