Crimean social democrats sent him to the capital, Petrograd, to represent them, but he soon moved to an internationalist revolutionary position, which made it impossible for him to remain in an organization dominated by less radical Mensheviks.

Immediately after the revolution, he supported Lenin and Trotsky against Zinoviev, Kamenev, Alexei Rykov and other Bolshevik Central Committee members who would have shared power with other socialist parties.

[clarification needed] From 30 November 1917 until January 1918, Joffe was the head of the Soviet delegation that was sent to Brest-Litovsk to negotiate an end to the hostilities with Germany.

[a] Once the Bolshevik Central Committee decided to sign the Treaty of Brest-Litovsk on 23 February 1918, Joffe remained a member of the Soviet delegation only under protest and in a purely consultative capacity.

The thus liberated nations then have to be brought to love each other [...] I advised him that we would not attempt to imitate the Russian example and that we likewise would not tolerate a meddling in our internal affairs.

Then he continued in a – for me, ever unforgettable – friendly, or I would even nearly say suppliant, tone: 'I very much hope that we will also be able to raise the revolution in your country...'[5]At the VII Extraordinary Congress of the Bolshevik Party between 6 and 8 March 1918, Joffe was re-elected to the Central Committee, but only as a candidate (non-voting) member.

On 6 November 1918, shortly before the Armistice and the German Revolution, the Soviet delegation in Berlin headed by Joffe was expelled from the country on charges of preparing a Communist uprising in Germany.

Straight before Joffe left Berlin he rendered Oskar Cohn about 1 million Mark and a 10.5 million Russian ruble mandate for a bank account at Mendelssohn & Co. After the delegation returned to Russia Joffe claimed to have paid this money to the Independent Social Democratic Party of Germany (USPD) to support the revolutionary activities and to purchase weapons.

[6][7] In 1919–1920, Joffe was a member of the Council of Labor and Defense and People's Commissar (minister) of State Control of the Ukrainian Soviet Republic.

In 1921 he signed the Peace of Riga with Poland, ending the Polish-Soviet War of 1918-1921, and was made deputy chairman of the Turkestan Commission of the VTsIK and Sovnarkom.

In 1923, Joffe signed an agreement with Sun Yat-Sen in Shanghai on aid to the Kuomintang on the assumption that the latter would cooperate with Chinese Communists, presumably with Lenin's approval.