Can I countersue the debt collector suing me in California?

In the right circumstances, yes, and when it applies, it can flip the entire dynamic of your case. There's a real list of triggers here: pursuing something past its legal shelf life, getting the numbers wrong on purpose, or reaching out again once you've put a stop-contact request in writing, any one of those can put them on the wrong side of federal or state collection law.

Both the federal statute and the state's own version attach a dollar penalty beyond whatever you actually lost, and both push legal costs onto the side that comes out behind. That last part changes the math meaningfully, a real counterclaim means the plaintiff now has something to lose too, not just you, which tends to make settlement conversations move differently.