Place the chicken thighs in the slow cooker.
In a medium bowl, whisk together the soy sauce, brown sugar, rice vinegar, garlic, ginger, and sesame oil until well combined.
Pour the sauce over the chicken in the slow cooker, ensuring the chicken is well coated.
Cover and cook on low for 6-7 hours or on high for 3-4 hours, until the chicken is tender and cooked through.
Once cooked, remove the chicken from the slow cooker and shred it using two forks.
In a small bowl, mix the cornstarch and water to create a slurry. Stir this into the remaining sauce in the slow cooker.
Turn the slow cooker to high and cook for an additional 10-15 minutes, or until the sauce thickens.
Return the shredded chicken to the slow cooker and stir to combine with the thickened sauce.
Serve the teriyaki chicken over rice or noodles, garnished with chopped green onions and sesame seeds.