In a small bowl, combine the rice vinegar, sugar, sesame oil, red pepper flakes, and salt. Set the dressing aside to give it time to blend.
Peel and slice the cucumber. Place the sliced cucumbers in a large bowl.
Chop the peanuts into smaller pieces, if desired. Slice the green onions.
Add the dressing, peanuts, and green onions to the sliced cucumbers. Stir to combine. Serve immediately or refrigerate until ready to eat. Give the salad a brief stir before serving to redistribute the dressing and flavors.