The word for ‘serious’ in French is « sérieux » for masculine objects and « sérieuse » for feminine objects.
In French, adjectives agree in gender and number with the nouns they modify. So if you are describing a serious man, you would say Un homme sérieux, and for a serious woman, it would be Une femme sérieuse.
Usage can vary depending on context, but this should cover most situations where you need to communicate the idea of seriousness in the French language.