* Russia (the most obvious one!)
* Belarus
* Kazakhstan
* Kyrgyzstan
* Tajikistan
* Ukraine (although this is contested due to the ongoing war)
It's also an official language in some regions of Moldova and Transnistria.
Keep in mind that while Russian is an official language in these countries, it doesn't mean everyone speaks it fluently or that it's the only language spoken there. Many other languages are spoken in these countries, and some regions may have a more prominent usage of Russian than others.