How to say "unexpectedly" in Japanese

• 案外あんがいJLPT N2approx.Common
  adverb (with と)The default choice when 'unexpectedly' means 'contrary to expectation': something turns out different from what you assumed, often better or easier than you thought, as when a dish you expected to dislike tastes good.
  ピクニックは案外あんがいつまらなかった。↗

  The picnic was a disappointment.
• 突如とつじょJLPT N1approx.Common
  adverb (with と), taru-adjectiveCovers 'suddenly; all of a sudden': something happening abruptly, with no lead-up. When 'unexpectedly' describes a surprising degree or outcome rather than a sudden occurrence, it is seldom the word you want.
  トムの怒いかりは突如とつじょ燃え上もえあがった。↗

  Tom's anger blazed out suddenly.
• にわかにCommon
  adverbCenters on 'suddenly; abruptly': a situation changing quickly from one moment to the next, such as weather turning. Usually written in kana alone; for 'unexpectedly' meaning 'surprisingly', it is only marginal.
  にわかに雨あめが降ふってきた。↗

  Suddenly, the rain fell.
• 不意にふいにCommon
  adverbCovers 'unexpectedly' as 'without warning, catching someone off guard' — being asked a question out of the blue, say. The emphasis falls on the surprise to the person affected rather than on speed of change.
  彼かれはふいに私わたしたちの話はなしに口くちをはさんだ。↗

  He interrupted us abruptly.
• 割とわりとJLPT N2approx.Common
  adverbReally means 'comparatively; fairly; rather'. A hint of 'more than expected' is why it appears here, but as a rendering of 'unexpectedly' it is marginal — choose it when 'fairly' is what you mean.

Choose 案外 for outcomes that defy expectations, 「不意に」 for things that catch you off guard, 突如 or 「にわかに」 for sudden changes, and treat 「割と」 as meaning 'fairly, relatively' rather than 'unexpectedly'.

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

• 案外あんがいunexpectedly
• 突如とつじょsuddenly
• にわかにsuddenly
• 不意にふいにsuddenly
• 割とわりとcomparatively
• たまたまby chance
• ふとsuddenly
• 割にわりにcomparatively
• 思いのほかおもいのほかunexpectedly
• 思いがけずおもいがけずunexpectedly
• 図らずもはからずもunexpectedly
• はしなくもunexpectedly
• その割にはそのわりにはunexpectedly
• ゆくりなくunexpectedly
• 期せずしてきせずしてunexpectedly
• たまさかcoincidentally
• 存外ぞんがいbeyond expectation
• ふらりaimlessly
• ひょいとsuddenly
• ぬっとsuddenly (appear, stick out, etc.)
• ふいとabruptly
• なかなかどうしてconsiderably (beyond one's expectations)
• のっとsuddenly (appear, stick out, etc.)
• にゅっとsuddenly (stick out, appear, extend, etc.)
• ひょっこりall of a sudden
• 飄然ひょうぜんcasually (come or go)
• ふらっとaimlessly
• 藪から棒にやぶからぼうにout of the blue
• 偶然ぐうぜんby chance
• よくもsurprisingly
• 急にきゅうにsuddenly
• ばったりunexpectedly (meeting someone)
• ぶらりunexpectedly
• ぱったりunexpectedly (meeting someone)
• ことのほかunexpectedly
• ぽっかりsuddenly
• ほっかりsuddenly
• ぷいとabruptly
• 何気になにげにtruthfully
• まさかindeed