Vsevolod Nikolayevich Merkulov (Russian: Всеволод Николаевич Меркулов; 27 November [O.S. 25 October] 1895 – 23 December 1953) was the head of NKGB from February to July 1941, and again from April 1943 to March 1946. He was a leading member of what was later derisively described as the "Beria gang".

Merkulov was born in 1895 in Zagatala in present-day Azerbaijan. His Russian father and Georgian mother were both minor members of the nobility. His father, an army captain, was convicted of embezzlement around the year 1900, and died in 1908. In 1913, Vsevolod Merkulov graduated from the Tiflis Gymnasium with a gold medal and became a student at Saint Petersburg University, Department of Physics and Mathematics, but did not graduate. In 1916, he was drafted into the Russian Army. In 1918, he left the army and moved to Tiflis (Tbilisi), where for a time he was unemployed, then worked as a teacher. From September 1921, after the Red Army invasion of Georgia, he was enlisted in Cheka, and its successor, the GPU.

Merkulov first met Lavrentiy Beria in 1923, when Beria was 24 and had been made deputy chairman of the Georgian GPU. He owed his subsequent promotions to Beria's patronage. He joined the Communist Party in 1925. From 1925 to 1931, he was Head of Secret Operations Directorate and Deputy Head of the GPU of Adzharistan. In 1931, he was made head of the Secret Political Department of the Transcaucasian GPU, but soon afterwards transferred to party work, when Beria was First Secretary of the Transcaucasian communist party. He wrote a pamphlet about Beria, entitled The Faithful Son of the Leninist–Stalinist Party. He was head of the Industry and Transport section of the Georgian CP Central Committee in 1936–38.

Merkulov was transferred to Moscow in August 1938, shortly after Beria had been chosen by Stalin to take over control of the NKVD from Nikolai Yezhov. When Beria took over as head of the Main Directorate of State Security (GUGB) on 29 September 1938, he chose Merkulov as his deputy. In 1939, he was elected a member of the Central Committee of the Communist Party of the Soviet Union

Merkulov interrogated Yefim Yevdokimov, an associate of Yezhov who had been under arrest for five months but had refused to cooperate. On 13 April 1939, Yevdokimov broke down and 'confessed' to being part of the 'Yezhov conspiracy'. At his trial, he said that he had confessed because he could not stand the pain of being beaten on his heels.

In February 1941, the NKVD was divided in two, and Merkulov was appointed Chairman of the NKGB. On 21 July 1941, his department was merged again with the NKVD, with Merkulov was Deputy People's Commissar, under Beria. On 20 July 1943, two departments were separated again, and Merkulov was again head of the NKGB.

In September 1939, following the pact between the USSR and Nazi Germany, Merkulov was sent to Ukraine to supervise the incorporation of territory seized from Poland. On 28 September, he reported to Beria that 1,923 people had been arrested, most of whom were accused of being Ukrainian nationalists. When almost 22,000 Polish officers were executed in the famous Katyn massacre in spring 1940, Merkulov headed the 'troika' who signed off the death sentences. Before it began, he visited the Belorussian Soviet Socialist Republic on 5 March, to check on progress in rounding up Poles trapped in the western part of the republic recently seized from Poland.

In May 1941, Merkulov was in charge of the pacification of the Baltic states, which were being forcibly incorporated in the USSR. On 17 June, he sent a reporting saying that he had had 14,467 people arrested in the three republics, and another 25,711 (mostly families of those arrested) evicted from their homes. It was calculated that in the short period before the German invasion, four percent of Estonia's population, and 2 per cent of the populations of Latvia and Lithuania were deported.