After Leningrad stabilized, Zhukov was sent to the Reserve Front on 8 October 1941. At that time, the USSR's situation was critical: the Battle of Moscow was taking place, and nearly the entire Soviet Western Front was being encircled in what eventually became a huge "cauldron battle" at in the Rzhev–Vyazma salient, where it was estimated that some 775,000 Soviet personnel were lost. Its commander, S. M. Budyonny, wasn't even present at headquarters and the officers in the High Command didn't seem to know what was happening at the front. An enraged Zhukov was thus forced to go to the front lines himself to grasp the battlefield situation, and then to search for Budyonny. To unify the operations of the huge numbers of Soviet forces, he sent a suggestion to Stalin that the Reserve and the Western Fronts be merged. After that, Zhukov became the de facto leader of the forces defending the Soviet capital city.
After a brief period, Zhukov established communication links with the encircled Soviet troops of the Western Front. After analyzing the situation and pointing out strengths and weaknesses of the German troops surrounding them, he gave specific instructions to their commanders and political personnel. Unable to break the Kessel (Ger. "kettle", or encirclement area), the surrounded Soviet troops did manage to strengthen their positions under Zhukov's leadership. Their efforts to some extent wore out some German units and thereby reduced the overall striking power of the offensive.
On 15 November, the Germans launched another attack on Moscow. At Krasnaya Polyana and Kryukovo, northwest of the capital, the Germans advanced to about 20 km from Moscow. Zhukov recognized an important error in the German plan—while the German forces seemed to attack aggressively from both their flanks, those in the center remained relatively inactive. From this observation, Zhukov made a rather daring decision: he ordered the repositioning of many of his centrally-located battle forces to reinforce his two flanks. With this tactical change, the Soviets stopped several German attacks with few losses among their reserve troops. Later on, these better-rested reserve forces played an important role in the counteroffensive.
Zhukov reasoned that the Germans would realize that this tactical scheme was problematic and would begin to attack in the now-weakened center. Zhukov therefore ordered the remaining forces in the center to prepare for an offensive. Just as he had predicted, the Germans began to attack the central sector troops. The Soviet preparations, however, managed to stop the German offensive.
After intense fighting, Moscow remained under Soviet control, while the German forces were exhausted and had lost equipment and supplies, a critical weakness given the long logistical tail. Although the Soviet combat forces were in no way superior compared to their German foes, Zhukov decided to launch his counteroffensive. On 1 December Zhukov was coordinating the Western, Bryansk, and the Kalinin Fronts preparatory to the counteroffensive. On 6 December the Soviet forces began a massive assault. After two months of bloody and brutal fighting, the Soviets pushed their German foes between 100 and 250 km away from Moscow—in some areas, up to 400 km—and had taken approximately 582,000 German soldiers out of action. This battle was the first time up to that point in World War II that the German army had been defeated in a large-scale battle involving millions of soldiers.
Ultimately, Operation Barbarossa failed. Perhaps most importantly, the great Soviet stand, counterattack, and ultimate victory at Moscow convinced the Allies that they could win. Zhukov received widespread accolades as the "savior of Moscow". Even Stalin heaped praise on Zhukov:
The Motherland and the Party will never forget the action of the Soviet commanders in the Great Patriotic War. The names of the victorious generals who saved the Motherland will forever be engraved in the honorary steles placed at the battlefields. Amongst these battlefields, there is one battlefield with exceptional meanings, and that is the great one at Moscow. And the name of Comrade Zhukov, as a symbol of victory, will never be apart from this battlefield.