Yes. The Quran sets a severe punishment for those who accuse chaste people without four witnesses (24:4), and curses those who slander believing women who never thought of evil (24:23). It is a major sin.
E àqueles que difamarem as mulheres castas, sem apresentarem quatro testemunhas, infligi-lhes oitenta vergastadas enunca mais aceiteis os seus testemunhos, porque são depravados.
Em verdade, aqueles que difamarem as mulheres castas, inocentes e fiéis, serão malditos, neste mundo e no outro, esofrerão um severo castigo.
This is general information, not a fatwa. For your own situation, ask a qualified scholar.
Questions about halal and haram
Is falsely accusing someone of immorality haram?
Yes. The Quran sets a severe punishment for those who accuse chaste people without four witnesses (24:4), and curses those who slander believing women who never thought of evil (24:23). It is a major sin.