Namajunas was born in Milwaukee, Wisconsin on , to Lithuanian parents who had moved to the United States in September 1991. She was named Rose () in honor of her great-grandmother Rožė Gotšalkaitė Namajūnienė, whose husband Juozas was an Independent Lithuania military officer. He enlisted in the Lithuanian military in 1933, and was promoted to lieutenant in 1936. During the Soviet invasion of Lithuania in 1939, he fought in the resistance. After the Soviets occupied the country, Juozas was forced to enlist in the Red Army. Following the German declaration of war on the Soviet Union and the subsequent Soviet retreat from the Baltic states, Juozas left the Red Army and retired to civilian life. When the Soviets re-occupied Lithuania, Juozas was arrested by the NKVD and sent to a prison camp. He was killed by Soviet KGB agents near his home in 1968. Namajunas's grandfather Algimantas Andriukonis was a successful wrestler, and won national championships in Lithuania and competitions in the USSR. Namajunas regularly visits Lithuania and communicates in the Lithuanian language with her grandparents.