How to say "coincidentally" in Japanese

• たまさか
  adverbCovers 'coincidentally' as something happening unexpectedly, by sheer chance or accident — a chance encounter that takes you by surprise. It is less common and usually written in kana alone, so seldom the word you want.
• 偶然にもぐうぜんにも
  adverbCovers 'coincidentally' and 'as it happens': it presents something as happening by chance rather than by design, often when the speaker finds the coincidence striking — say, meeting an acquaintance far from home.
  私は偶然ぐうぜんにも京都きょうとで旧友きゅうゆうにあった。↗

  I met an old friend by chance in Kyoto.

For plain 'coincidentally' — something happening by chance rather than by design — 「偶然にも」 is the straightforward choice, while 「たまさか」, which also covers 'unexpectedly' and 'accidentally', is a less common, kana-written alternative.

Usage comparisons are AI-assisted and cross-checked by multiple models; for study reference only

• たまさかcoincidentally
• 偶然にもぐうぜんにもby chance