The flag's design was based on the flag of the former Soviet Union, with the additional horizontal blue (1:6 width), and green (1:5 width) bands added at the bottom.

At the top of the flag near the flagpole were a golden hammer and sickle and the red star with a gold border.

The green color symbolized the forest resources, and blue represented the abundance of rivers and lakes.

In April 1940, the flag was created, which was a red background with a gold hammer and sickle in the top-left corner, and the Latin characters Karjalais-Suomalainen SNT (Karjalais-Suomalainen Sosialistinen Neuvostotasavalta) with Карело-Финская ССР (Карело-Финская Советская Социалистическая Республика) below them in gold in a sans-serif font.

[4] In 1956, Karelia became an autonomous republic of the RSFSR again and had to change its flag, which was adopted on 10 October 1956.