I think it's unethical if your reason for linking is financially motivated, but it's not obvious from the context. In that case you're intentionally deceitful.

If you would have linked to it regardless of the financial incentive (i.e. even if they didn't have an affiliate program), then I'm not sure I'd consider it unethical, but I still highly recommend disclosing it.

I don't tend to use affiliate links because they come across disingenuous even if they are not. In the exceptions I do use affiliate links, I clearly disclose it and often add a non-affiliate link as well so people have an option.