Cook the spinach in a large pot of boiling salted water for 30 seconds, or until just tender. Drain the spinach in a colander and rinse with ice water. Blot the spinach leaves dry on paper towels or a dish towel.
Lightly toast the sesame seeds in a dry skillet over medium heat for 2 minutes. Combine half the sesame seeds and the sesame oil, soy sauce, and sugar in a bowl. Whisk until the sugar is dissolved.
Add the spinach and toss well. Correct the seasoning, adding more soy sauce or sugar to taste: The salad should be a little sweet and a little salty. Sprinkle the spinach with the remaining sesame seeds and serve at room temperature or chilled.