Bring a pot of water to a boil. Carefully add the eggs and boil for 7 minutes. Transfer immediately to cold water, cool completely, peel, and pat dry.

In a saucepan, combine soy sauce, mirin, water, sugar, and rice vinegar. Bring to a brief boil to evaporate the alcohol from the mirin. Remove from heat and allow to cool slightly.

Add the peeled, dried eggs to the marinade. Refrigerate for 24 hours, flipping halfway through if the eggs are not fully submerged.

After marinating, store in the refrigerator for up to 2 additional days.