Yes, Within the Framework of Civil (Not Criminal) Law
In the United States, online defamation is generally addressed as a civil matter — meaning the person harmed can sue for damages — rather than as a crime prosecuted by the state, though a small number of states retain criminal libel statutes that are rarely used. So "illegal" in the everyday sense of "you could be sued for it and lose" is accurate; "illegal" in the sense of "the police will arrest the poster" usually is not, except in narrower circumstances like harassment, stalking, or true threats that accompany the defamatory content.
Terminology Worth Knowing
Libel is defamation in a fixed form (written, printed, posted, recorded); slander is spoken defamation. Because almost everything online exists in some fixed, recorded form, online defamation is treated as libel. You may also see the term "cyber tort" used informally to describe civil wrongs committed online, including defamation — it's a useful shorthand in some articles and course materials, but it isn't a distinct legal cause of action with its own elements. A false, damaging online statement is analyzed under ordinary state defamation (libel) law.
Civil Remedies Are the Real Path
If you're dealing with defamatory content, the realistic path is a civil claim (a lawsuit for damages and, potentially, an injunction requiring removal), platform reporting under the platform's own content policies, or both — not a criminal complaint, except in the narrower cases involving accompanying threats or harassment, which is a separate conversation with law enforcement.