Does Mark Harmon know sign language?

There is no publicly available information confirming whether Mark Harmon knows sign language.

It's important to remember that:

* Celebrities' personal lives are often private: We don't have access to their personal skills and knowledge unless they choose to share it publicly.

* There's no connection between his acting roles and real-life skills: Just because Mark Harmon plays a character who knows sign language in a show doesn't mean he himself knows it.

So, unfortunately, we can't say for sure whether Mark Harmon knows sign language.