Will I be cold?
Will I be cold in Tokyo in December?
Verdict
Yes — Yes, cold.
Lows drop to 39°F on average — noticeably cold, especially after sundown.
Day vs. night
Day
54°F
Cool
Sweater weather with the option to add a light shell.
Night
39°F
Cold
Cold enough that a hat and gloves become mandatory.
Why
Tokyo in December averages highs of 54°F, lows of 39°F, 5 rainy days.
Plan for real winter kit — an insulated coat, warm hat, and gloves rather than a light jacket.