Boil the Eggs: Fill a saucepan with water and bring it to a boil. Gently lower the eggs into the boiling water. Cook them for about 7 minutes, then transfer immediately to a bowl of ice water. Let them chill for at least 5 minutes.
Peel the Eggs: Once the eggs are cool, crack them gently all over and peel under running water for best results.
Make the Marinade: In a bowl or measuring cup, stir together the soy sauce, water, mirin, and sugar until the sugar is dissolved.
Marinate the Eggs: Place the peeled eggs in a small container or zip-top bag and pour the marinade over them. Make sure the eggs are mostly submerged—if not, turn them occasionally. Refrigerate for at least 6 hours and up to 24 hours.
Serve or Store: Remove the eggs from the marinade and slice in half to serve. Store leftovers in an airtight container in the fridge for up to 3 days.
Notes
For a deeper flavor, marinate the eggs overnight, but no longer than 24 hours unless you prefer a very strong soy flavor.
You can reuse the marinade once more if you're making another batch within a couple of days—just bring it just to a boil and cool before reusing.
These are best served cold or at room temperature, but they’re also great dropped into hot ramen just before serving.