Ivar Tenisovich Smilga (Latvian: Ivars Smilga), Russian: И́вар Тени́сович Сми́лга (1892–1938) was a Bolshevik revolutionary leader in, and member of the Left Opposition in the Soviet Union.

Ivar was born in Aloja in the Governorate of Livonia (gubernia) (modern Latvia), as the son of a forester killed by Russian Government troops in 1906 during the last stage of the Russian Revolution of 1905.

Smilga was the Chairman of the Regional Committee of the Soviets in Finland in 1917, chairman of Tsentrobalt (i.e.: Central Committee of the Baltic Fleet), 1917-1918. In April 1917 he was elected to the Central Committee of the Bolshevik Party 

Along with Mikhail Tukhachevsky, he led the Seventh Army during the Polish-Soviet War in 1920. He was vice-chairman of the Vesenkha from 1921 to 1928, and of the Gosplan from 1924 to 1936.

Along with Yevgeni Preobrazhensky, he renounced his support for the Left Opposition, citing the reason that Joseph Stalin's rise would have meant the application of much of the Left's recommended policies, and that the dangers the Soviet state faced, from the outside as well as from within, required their "return to the Party". In 1929, however, he was expelled from the Central Committee and then from the Communist Party of the Soviet Union.

When he was banished to Khabarovsk in the Russian Far East (under the pretext of doing "party work" in the provinces), the authorities met with a spontaneous rally of about a thousand men, gathered at the railway station to demand his release.

He was arrested in 1935 after Kirov's assassination, imprisoned, and finally executed in 1938. He was posthumously exonerated in 1987.