seriös
reputable
Describes a person, company, or offer as trustworthy, credible, and honorable.
Das ist ein sehr seriöses Angebot, da gibt es keinen Haken.
That is a very reputable offer; there is no catch.

reputable / respectable
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reputable
Describes a person, company, or offer as trustworthy, credible, and honorable.
Das ist ein sehr seriöses Angebot, da gibt es keinen Haken.
That is a very reputable offer; there is no catch.

'Seriös' means 'trustworthy' or 'respectable'. It is not the same as the English 'serious', which often means 'ernst' (not laughing) or 'schwerwiegend' (a serious injury).
Ich suche eine seriöse Firma für die Renovierung meiner Wohnung.
I am looking for a reputable company to renovate my apartment.
In the meaning 'trustworthy', 'seriös' is often used for things in business life. Typical combinations are 'ein seriöses Unternehmen' (a reputable company), 'eine seriöse Quelle' (a reliable source), or 'ein seriöser Anbieter' (a reputable provider).
Prüfen Sie immer, ob die Website des Online-Shops seriös ist.
Always check whether the online shop's website is reputable.
When it refers to people, 'seriös' describes a serious and professional appearance. Common combinations are 'seriöse Kleidung' (respectable clothing), 'ein seriöses Auftreten' (a professional demeanor), or 'seriös wirken' (to appear respectable).
Für das Vorstellungsgespräch ist ein seriöses Auftreten sehr wichtig.
For the job interview, a professional demeanor is very important.
serious
'ernst' and 'seriös' can both mean that something does not seem funny or playful. 'ernst' is more likely to describe a person's mood, behavior, or facial expression. 'seriös' more strongly describes someone or something as trustworthy, credible, or appropriately formal, for example a company, an offer, clothing, or someone's appearance and manner. Learners confuse the words because 'seriös' often also seems serious. But 'ernst' does not automatically mean that someone is trustworthy. And 'seriös' does not simply mean sad or strict, but reliable and appropriate.

Die Ärztin wirkt sehr seriös und erklärt jeden Schritt genau.
The doctor seems very professional and explains every step carefully.
Sofia schaut ernst, weil sie sich auf die Prüfung konzentriert.
Sofia looks serious because she is concentrating on the exam.