The 2021 Kremlin Cup (also known as the 2021 VTB Kremlin Cup for sponsorship reasons) was a professional tennis tournament played on indoor hard courts. It was the 31st edition of the Kremlin Cup for the men and the 25th edition for the women. The tournament was part of the ATP 250 series of the 2021 ATP Tour, and of the WTA 500 series of the 2021 WTA Tour. It was held at the Irina Viner-Usmanova Gymnastics Palace and the Luzhniki Palace of Sports in Moscow, Russia, from 18 October through 24 October 2021.

This was Kontaveit's 4th WTA title, and 3rd of the year.

The following players received wildcards into the singles main draw:

The following players received entry using a protected ranking:

The following players received entry from the qualifying draw:

The following player received entry as a lucky loser:

The following pairs received wildcards into the doubles main draw:

The following players received wildcards into the singles main draw: