If Russia decided to print US currency, what will the US do?

Funny:

The official term for what you're suggesting is a Superdollar. If I'm not mistaken, North Korea employs a tactic of printing USD and the US doesn't have a specific set of sanctions against such action as we already have enough sanctions on the regime, I would imagine, since Russia is also already facing sanctions, nothing would change in official policy against Russia but we'd definitely condemn such action, and if it became too much of a problem even more USD will be made with the complex design of our $100 bill.

As someone who formerly worked in the banking industry, many of my clients when traveling abroad wanted the newest $100 bills as the place they were traveling would only accepted them. Therefore Russia could theoretically print more super dollars but we'd only make the USD more complicated. Plus it's not in Russia's best interest to commit to such an action as they hold about $300 billion USD in FOREX, they need a strong dollar as much as we do (though many, including myself, believe it should be weaker to one degree or another).

And yes, North Korea does circulate anywhere between $50 million to a billion annually in counterfeit therefore they serve as an excellent example.