Place the rice in a fine-mesh strainer and rinse under cold water until the water runs mostly clear.
Add the rinsed rice, water, and kombu, if using, to a medium saucepan. Let soak for 30 minutes.
Bring to a boil over high heat. Reduce the heat to low, cover, and simmer for 15 minutes.
Remove from the heat and let the rice stand, covered, for 10 minutes.
Meanwhile, combine the rice vinegar, sugar, salt, and soy sauce in a small saucepan. Heat over low heat, stirring until the sugar and salt dissolve. Remove from the heat and let cool slightly.
Transfer the cooked rice to a large, wide, nonmetallic bowl. Remove and discard the kombu.
Drizzle the vinegar mixture evenly over the warm rice.
Using a rice paddle or wooden spoon, gently fold and cut through the rice until the seasoning is evenly distributed. Avoid stirring aggressively or mashing the grains.
Continue gently folding the rice as it cools to room temperature.
Cover with a clean, damp kitchen towel until ready to use.
Video
Notes
Short-grain Japanese rice is best for sushi because it develops the proper tender, sticky texture. Avoid substituting long-grain rice.Rinse the rice thoroughly before cooking to remove excess surface starch and prevent a gummy texture.Kombu is optional and adds subtle savory flavor to the rice.For the best sushi texture, use the rice the same day it is prepared.Refrigerate leftovers in an airtight container for up to 5 days, but chilled rice will become firmer.