Short answer: Strictly, miso is not allowed in a traditional Jain diet.

Reason in simple terms:

• Miso is a fermented food (fermented soybeans with koji fungus/yeast, sometimes with added alcohol).
• In Jain maryada, fermented foods are generally avoided because:

- They involve intentionally growing huge numbers of microorganisms. - Later, many of these living beings are destroyed or harmed.

• Many commercial miso products are also used in miso soup with fish stock (dashi) or may contain alcohol / wine – both clearly non‑Jain.

From a traditional Jain viewpoint (both Shwetambar and Digambar):

• Soybeans themselves (non‑fermented) can be acceptable (like plain tofu, soya flour), if other rules are followed.
• Once fermented into miso, it usually falls into the “avoid fermentation” group, so a careful Jain would not eat it.

In practice:

• Some modern/lax Jains do eat certain fermented items (like a little soy sauce, curd, etc.), but this is leniency, not the ideal.
• For following shuddh Jain aahar, you should avoid miso and miso soup, and prefer fresh, non‑fermented preparations.

For a clear list of what Jains usually avoid and why (including fermented foods), you can read more here