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.
Na wanao wasingizia wanawake mahashumu, kisha wasilete mashahidi wane, basi watandikeni bakora thamanini, na msiwakubalie ushahidi wao tena. Na hao ndio wapotovu.
Hakika wanao wasingizia wanawake, wanao jihishimu, walio ghafilika, Waumini, wamelaaniwa duniani na Akhera, nao watapata adhabu kubwa.
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.