Is this sentence grammatically sound?

[VladmirsPoutine]

"Feminist are not only women."

I'm inclined to think it isn't and it should be "Feminists", however apparently if thought of as an adjective rather than a noun it's fine. Is this the case?

Well, it should be feminists, but the entire sentence sounds odd.

Men can be feminists, too, perhaps?

You are talking about feminism as an adjective-you are using it to describe someone, however you are using it to describe plurally so I’m sure it needs the s at the end.
It doesn’t sound right with no s

If it's "feminist" as an adjective, it should be "is". Otherwise "feminists".

“Feminism is not only limited to females”?

Sounds weird. It's also wrong

I think it should be either “feminists are not only women” or “not only women are feminist” - the second is using feminist as an adjective

I'm not the author of the sentence. It's on a t-shirt and I've been trying to get my head around it being correct which my brain won't accept. So it's categorically incorrect? I've been thinking about it all morning

The problem with 'only' and 'limited to' is that feminism frankly, isn't improved by men. So if you wanted to say this, and I wouldn't, much better would be 'feminism for all!' or 'everyone for women's rights'.

It's not correct. Why do you need to get your head around it? The word feminist isn't being used as an adjective, it's a noun in this sentence and is just wrong. It sounds like it was badly translated, like many t shirts - no need to try to justify it.

Yep, it's incorrect. If you were using feminist as an adjective, then it would be 'Not only women are feminist', which is still grammatically horrible. The 'are' in that case refers to women, not feminist, hence the plural.

You could also say 'Feminists are not only women', still horrible but in that case the 'are' refers to the plural noun 'feminists'.

It's wrong because while there are some parts of speech other than the subject that can be front-ended in English such as the direct object using a cleft sentence structure, adjectives can't.

Does it mean that feminists aren’t only women, they are also mothers, doctors, lawyers etc?