Will I be cold?
Will I be cold in Tokyo in January?
Verdict
Yes — Yes, cold.
Lows drop to 34°F on average — noticeably cold, especially after sundown.
Day vs. night
Day
50°F
Cool
Sweater weather with the option to add a light shell.
Night
34°F
Cold
Cold enough that a hat and gloves become mandatory.
Why
Tokyo in January averages highs of 50°F, lows of 34°F, 5 rainy days.
Plan for real winter kit — an insulated coat, warm hat, and gloves rather than a light jacket.