How to say "reason" in Japanese

Choosing the right word

訳わけJLPT N4approx.Common
nounThe reason behind a situation or turn of events: use it when explaining why something is or happened as it did, rather than a reason formally given for an action.
そういうわけで私はこんなに早はやく帰かえって来きたのです。↗
That's why I came back so soon.
理由りゆうJLPT N4approx.Common
nounThe word you want for most senses of 'reason': the grounds or motive you give, ask about, or state for an action, decision, or event; it can also shade toward 'pretext' or 'excuse'.
どうしてあなたにお詫わびする理由りゆうがあるのですか。↗
Why should I apologize to you?
由よしCommon
nounSeldom the word you want for 'reason': on its own it survives mainly in set phrases and formal writing, and is better thought of as 'significance' or 'cause'.
理りCommon
nounReason as abstract principle or logic, not a particular 'reason why' something happened; by itself it is seldom the word you want when giving a reason.
彼かれの言葉ことばは理りにかなっているようだ。↗
His words sound reasonable.
理性りせいJLPT N1approx.Common
nounReason as a faculty of the mind — the power to think and judge, contrasted with emotion or instinct; not used for 'the reason why' something happened.
人間にんげんには理性りせいがあるが動物どうぶつには無ない。↗
Man has reason, animals do not.

For 'the reason why' choose 理由, or 訳 when explaining a situation; 理性 means reason as a mental faculty, 理 abstract principle or logic, and 由 survives mainly in set phrases.

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

Japanese equivalents

訳わけreason
理由りゆうreason
由よしreason
理りreason
理性りせいreason
道理どうりreason
理屈りくつtheory
途方とほうway
条理じょうりreason
事由じゆうreason
筋道すじみちreason
筋合いすじあいreason
一理いちり(a) principle
根拠こんきょbasis
言い聞かせるいいきかせるto tell
ゆえreason
理ことわりreason
リーズンreason
所以ゆえんreason
いわれreason
事理じりreason
わけあいreason
原由げんゆreason
訳柄わけがらreason
結論づけるけつろんづけるto reason
説き聞かせるとききかせるto explain
由縁ゆえんacquaintance
言い聞かすいいきかすto tell
申し聞かせるもうしきかせるto tell
文理ぶんりreason
所為しょいcause
節理せつりreason
筋目すじめreason
縁由えんゆorigin
ものreason
次第しだいreason
筋すじreason
意趣いしゅreason

Say it right, write it right