Will I be cold?
Will I be cold in Tokyo in September?
Verdict
No — No, comfortable.
Lows stay near 68°F — you won't be reaching for the puffer.
Day vs. night
Day
80°F
Warm
Breathable fabrics; sunscreen matters more than a jacket.
Night
68°F
Comfortable
Comfortable outside without an extra layer.
Why
Tokyo in September averages highs of 80°F, lows of 68°F, 12 rainy days.
You can pack for the daytime temps and not worry about the evening dip.