Siaka Probyn Stevens (24 August 1905 – 29 May 1988) was the 3rd prime minister of Sierra Leone from 1967 – 1971 and the 1st president of Sierra Leone from 1971 – 1985. Stevens is generally criticised for dictatorial methods of government in which many of his political opponents were executed, as well as for mismanaging the economy. On a positive note, he reduced the ethnic polarisation in the government of Sierra Leone by incorporating members of various ethnic groups into the government.

Stevens was born on August 24, 1905 in Moyamba, Moyamba District, in the Southern Province of Sierra Leone to parents from the Limba ethnic group. He completed secondary school at the famous Albert Academy in Freetown, the same secondary school Sierra Leone's first Prime Minister Sir Milton Margai attended. He attended Ruskin College in Oxford, England, where he studied Trade Unionism. Stevens joined the Sierra Leone Police Force from 1923 to 1930. He rose to the rank of First Class Sergeant. Stevens and his All People's Congress (APC) party won the closely contested 1967 Sierra Leone general elections over the incumbent Prime Minister Sir Albert Margai of the Sierra Leone People's Party (SLPP). In April 1971, Stevens made Sierra Leone a republic and he became the first President of Sierra Leone a day after the constitution had been ratified by Parliament. Stevens served as Chairman of the Organization of African Unity (OAU) from 1 July 1980 to 24 June 1981, and engineered the creation of the Mano River Union, a three country economic federation of Sierra Leone, Liberia and Guinea. Stevens retired from office at the end of his term on 28 November 1985. After pressuring all other potential successors to step aside, he chose Major-General Joseph Saidu Momoh, the commander of the Sierra Leone Armed Forces as his successor.

Siaka Probyn Stevens was born on August 24, 1905 in Moyamba, Moyamba District, in the Southern Province of Sierra Leone, to Limba parents. Although born in Moyamba, Stevens was largely raised in Freetown. Stevens completed his primary education in Freetown and completed secondary school at Albert Academy in Freetown, before joining the Sierra Leone Police Force. From 1923 to 1930, Stevens rose to the rank of First Class Sergeant and Musketry Instructor.

From 1931 to 1946, he worked on the construction of the Sierra Leone Development Company (DELCO) railway, linking the Port of Pepel with the iron ore mines at Marampa. In 1943, he helped co-found the United Mine Workers Union and was appointed to the Protectorate Assembly in 1946 to represent worker interests. In 1947, Stevens studied labor relations at Ruskin College.

In 1951, Stevens co-founded the Sierra Leone People's Party (SLPP) and was elected to the Legislative Council. A year later, he became Sierra Leone's first Minister of Mines, Lands, and Labor. In 1957, he was elected to the House of Representatives as a member for Port Loko constituency, but lost his seat as a result of an election petition.

After disagreements with the SLPP leadership, Stevens broke ties with the party and founded the People's National Party (PNP), of which he was the first secretary general and deputy leader. In 1959, he participated in independence talks in London. When the talks concluded, however, he was the only delegate who refused to sign the agreement on the grounds that there had been a secret defense pact between Sierra Leone and the United Kingdom. Another point of contention was the Sierra Leonean government's position that there would be no elections held before independence, which would effectively shut him out of the political process. He was promptly expelled from the PNP upon his return from the talks. Stevens then launched the Elections Before Independence Movement (EBIM), which was later transformed into the All People's Congress (APC).

After successfully exploiting the disenchantment of northern and eastern ethnic groups with the SLPP, along with the creation of an alliance with the Sierra Leone Progressive Independence Movement (SLPIM), the APC became the main opposition party following elections held in 1962. Stevens was later elected mayor of Freetown.

In elections held on 17 May 1967, the APC won by an extremely narrow margin, and Stevens was appointed Prime Minister, but he was arrested in only an astonishing several minutes after taking office during a military coup.

After a brief period of military rule, Stevens reassumed the post of Prime Minister on 26 April 1968. In April 1971, a republican constitution was introduced. It was ratified by the House of Representatives on 20 April. A day later, Stevens became the country's first president, with wide executive and legislative powers.

In 1973, the first elections under the new constitution were held. The polls were marred by violence and were boycotted by the SLPP, which gave the APC all 85 seats in the House of Representatives. In March 1976 Stevens was re-elected President unopposed by the House. Stevens's vice-president from 1971 until leaving office in 1985 was Sorie Ibrahim Koroma.

Throughout the remainder of the 1970s, Stevens continued to consolidate his power, which culminated in a 1978 referendum on a new constitution that would create a single party state. However, the country had been a de facto one party state since Sierra Leone became a republic. On 12 June, 97.1% of voters were reported to have voted for the new one party constitution, an implausibly high total that could have only been obtained by massive fraud. Observers agreed that the elections had been heavily manipulated by the government. Proving this, even areas where the SLFP was still dominant were reported as supporting the one party state by landslide margins. Stevens billed the new one party system as more "African" than Western style democracy.

Following the election, all opposition members of the House of Representatives were required to join Stevens's APC or lose their seats. Two years after being re-elected for a five year term, Stevens was sworn in for an additional term of seven years, having by then adopted the title of "Dr."

President Stevens served as Chairman of the Organization of African Unity (OAU) from 1 July 1980 to 24 June 1981, and engineered the creation of the Mano River Union, a three country economic federation of Sierra Leone, Liberia and Guinea.

Stevens' regime was very repressive and corrupt, even by African standards of the time. Many of his opponents, some of which were once close associates, were imprisoned and killed. The Internal Security Unit, a gang of unemployed urban youths amply supplied with drugs, was deployed as Stevens' personal death squad. Among his close associates sent to the gallows were John Amadu Bangura, who had once plucked Stevens from political oblivion when the army obliterated civilian politics after the 1967 Huha elections; at that time, Stevens had been down and out, living in exile in Conakry, Guinea, with his main remaining option, a planned assault on the sovereignty of Sierra Leone and her citizens. Bangura was to be the ring leader, but the plan never materialized because of a coup headed by Bangura. Bangura, in turn, handed over power to Siaka Stevens as prime minister. Another prominent Sierre Leonean murdered during Siaka Steven's rule was Dr Mohamed Forna. He was hanged along with 14 other people in 1974 after trumped up charges of treason. Dr Forna was the popular finance minister when Steven's came to power. He had fallen out of favor after protesting about rampant corruption.

Stevens also grossly mismanaged the economy. He and his closest colleagues looted state resources, to the point that the state was unable to supply basic services. The education system was more or less nonexistent. The poverty was especially pronounced in rural areas, which were largely isolated from Freetown.

Stevens retired from office at the end of his term on 28 November 1985. After pressuring all other potential successors to step aside, Major General Joseph Saidu Momoh was sworn in as the new President of the Republic.

He died on 29 May 1988 in Freetown.