危ない vs 危険
JLPT N5 vocabulary · what is the difference?
The meaning is nearly the same; the register is not. 危ない is the everyday spoken word — 危ない！ is what you shout to warn someone. 危険 is the formal written word you see on signs, in news reports and in official notices.
When to use each
危ない
あぶない
Use 危ない in conversation, and as an immediate shout of warning. It also covers "that was close" — 危なかった after a near miss.
危ない！車が来るよ。
あぶない！くるまがくるよ。
Look out! A car's coming.
危ないですから、走らないでください。
あぶないですから、はしらないでください。
It's dangerous, so please don't run.
危険
きけん
Use 危険 in writing, signage and formal speech. As a noun it also names the danger itself: 危険がある, 危険を知らせる. On a sign, a single 危険 means DANGER.
ここで泳ぐのは危険です。
ここでおよぐのはきけんです。
Swimming here is dangerous.
この工事現場は危険です。
このこうじげんばはきけんです。
This construction site is dangerous.
Sentences where only one works
危ない！
あぶない！
Watch out!
A sudden warning is always 危ない. Shouting 危険 at a friend would sound like reading a sign aloud.
危険、立入禁止。
きけん、たちいりきんし。
Danger — no entry.
Signs use the noun 危険. 危ない would look strangely chatty on a notice.
この薬には危険があります。
このくすりにはきけんがあります。
This medicine carries a risk.
Here danger is a thing you can have, and only 危険 is a noun. 危ない cannot take があります.
その橋は古くて危ないよ。
そのはしはふるくてあぶないよ。
That bridge is old and dangerous.
Casual speech to a friend takes 危ない.
Common mistakes
危険！
危ない！
As a spoken shout of warning, Japanese uses 危ない. 危険 is the label, not the cry.
危ないがあります。
危険があります。
危ない is an い-adjective and cannot be the subject of あります. Only the noun 危険 works there.
危ないな場所
危ない場所
危ない is an い-adjective, so it takes no な. The な-adjective is 危険, as in 危険な場所.
Frequently asked questions
Are they ever interchangeable?
Often, yes. 危ない場所 and 危険な場所 both work and mean the same thing; only the tone differs. The distinction is mostly register, not meaning.
Why do signs never say 危ない?
Written and official Japanese prefers kanji compounds. 危険 is short, formal and instantly readable, which is exactly what signage wants.
What does 危なかった mean on its own?
"That was close!" — said after narrowly avoiding something. 危険でした would just be a flat statement that it was dangerous.