Add all the ingredients to a saucepan. Place the pan over medium heat and bring the mixture to a gentle boil, stirring with a wooden spoon to dissolve the sugar. This should take about 4–5 minutes. Keep the pan uncovered to prevent it from boiling over.
Once the sauce reaches a boil, reduce the heat to medium-low and let it simmer until it thickens slightly, about 10 minutes. Stir occasionally to ensure it doesn’t stick to the saucepan.
When the sauce has thickened, remove the pan from the heat and set it aside to cool. Transfer the cooled sauce to a jar for storage. Note that the sauce will thicken further as it cools. The final consistency will be glossy and liquid-like—not overly thick or syrupy, but with a nice sheen and a slightly viscous texture.
Notes
Flavor ratio: To achieve authentic teriyaki sauce flavor, use this ratio: 2 parts mirin : 2 parts sake : 2 parts soy sauce : 1 part brown sugar (2:2:2:1).