The date, opening what is referred to in both countries as the Great Patriotic War, is observed as the Day of Memory and Grief in Russia and Belarus.
Medvedev laid wreaths at the Eternal Flame at the Brest Fortress with Belarussian President Alexander Lukashenko, where he visited the site's museum and met with veterans, the Kremlin web site reported.
Earlier in the day, he laid a wreath at the Tomb of the Unknown Soldier in Moscow, Interfax reported.
Medvedev called the choice of Brest as a meeting place "a symbol, a good sign," RIA-Novosti reported.
Lukashenko said Medvedev's visit to Brest was "a spiritual, human step," and called the fortress "our shared holy place," Interfax reported.
Brest is a symbolic spot as the first on Soviet territory to meet the invaders.
The Nazis handed the city over to the Soviet Union in 1939, following the invasion of Poland, of which the city was then part.
At the outset of Germany's surprise attack on the Soviet Union on June 22, 1941, the fortress was surrounded, with Soviet troops holding out for a month before surrendering. The city was recaptured by Soviet troops in 1944.
A memorial complex was opened there in 1971.
Medvedev wrote in the museum visitor's book that the defense of the fortress was "unparalleled in history" and that its memory was a "guarantee of brotherly friendship" between the two countries, Interfax reported.
The two leaders also held one-to-one talks. Afterward, Medvedev called the relationship between the two countries "a strategic partnership," and said "in general, we are moving ahead very well."
He called for the 1999 agreement to form a unified state to be filled with "concrete content," Interfax reported.
A Message from The Moscow Times:
Dear readers,
We are facing unprecedented challenges. Russia's Prosecutor General's Office has designated The Moscow Times as an "undesirable" organization, criminalizing our work and putting our staff at risk of prosecution. This follows our earlier unjust labeling as a "foreign agent."
These actions are direct attempts to silence independent journalism in Russia. The authorities claim our work "discredits the decisions of the Russian leadership." We see things differently: we strive to provide accurate, unbiased reporting on Russia.
We, the journalists of The Moscow Times, refuse to be silenced. But to continue our work, we need your help.
Your support, no matter how small, makes a world of difference. If you can, please support us monthly starting from just $2. It's quick to set up, and every contribution makes a significant impact.
By supporting The Moscow Times, you're defending open, independent journalism in the face of repression. Thank you for standing with us.
Remind me later.