Quick answer
Ты серьёзно?
ty seryózna — the adverb, not the adjective — and the shocked versions
The options
Ты серьёзно?Are you serious? — informal
Вы серьёзно?Are you serious? — polite
Да ла́дно!No way! · Come on!
Пра́вда?Really?
Не́ужели?Surely not? — genuine disbelief
Why серьёзно
It is the adverb, describing *how* you are speaking — not an adjective describing you. Ты серьёзный would mean “you are a serious person”. The same logic gives Э́то серьёзно — this is serious.
The scale of disbelief
Пра́вда? — mild, genuine interest. Серьёзно? — surprised. Да ла́дно! — informal, “get out of here”. Не́ужели? — slightly bookish, strong disbelief. Шу́тишь? — you’re joking, right?
What you hear back
Абсолю́тно серьёзно.Completely serious.
Нет, шучу́.No, I’m joking.
Че́стное сло́во!Honestly!
Сам не ве́рю.I can’t believe it myself.
Typical mistakes
Ты серьёзный?
Ты серьёзно?
The adverb describes the statement, not the person.
Не́ужели — in a casual chat
Да ла́дно!
Не́ужели is noticeably more formal.
Check yourself
Which is correct?
The adverb серьёзно.