How to Say "Of Course" in Japanese

The everyday way to say "of course" in Japanese is もちろん (mochiron), which works in most friendly and neutral situations. For politeness add です; for a blunter "obviously" use 当たり前, and in formal writing or speeches use 無論.

The most common way

mochiron

Of course — a friendly, neutral "yes, that's obvious," used to agree or confirm.

How to say it: Say it as four even beats, mo-chi-ron, with no stress on any one syllable; English speakers tend to rush the middle "chi" and make it too short.

Other ways to say it

polite
mochiron desu
Of course — the same friendly certainty, wrapped in a respectful ending.
When to use it: To a boss, a customer, or someone you have just met, in an ordinary polite conversation.
casual, blunt
atarimae desho
Obviously / that goes without saying — blunt and a little "duh."
When to use it: Only with close friends or family, where a teasing or dismissive tone will not offend.
formal, written
muron
Of course — a formal, slightly literary way of stating something as logically obvious.
When to use it: In speeches, essays or formal writing, rather than in everyday chat.
responding to a request
yorokonde
Gladly — of course, I'd be happy to do it.
When to use it: When agreeing to do something for someone, such as a favor or a small request.
formal, needless to say
iu made mo naku
Needless to say — used to set out an obvious point before making your real one.
When to use it: In presentations, formal explanations or writing, when introducing a fact everyone already accepts.

In a conversation

A
、。
Ashita no kaigi, koraremasu ka.
Can you come to tomorrow's meeting?
B
。
Mochiron desu.
Of course.
A
。
Shiryō mo onegai dekimasu ka.
Could you also take care of the materials?
B
。
Yorokonde.
Gladly.

Common mistakes

~~当たり前です~~ → もちろんです
当たり前です sounds like "that's obvious, duh" and can come across as rude or condescending to a superior.
~~もちろんないです~~ → もちろん違います
For "of course not," Japanese negates with 違います rather than attaching ない to もちろん.