Konstantinos Simitis (Greek: Κωνσταντίνος Σημίτης; 23 June 1936 – 5 January 2025) was a Greek politician who led the 'Modernization' movement of Greece. He succeeded in leadership Andreas Papandreou, the founder of the Panhellenic Socialist Movement (PASOK), and served as Prime Minister of Greece from 1996 to 2004.

Simitis was a founding member of PASOK, and he gained significant ministerial experience in Papandreou's governments. Simitis' reputation was marked when he became Minister of National Economy in 1985 when PASOK's profligacy needed a new financial 'stability' imposed through an economic adjustment programme. With stability achieved and the party's popularity waning, Papandreou distanced himself from Simitis' policies, and Simitis resigned.

In 1996, Simitis won the leadership of PASOK after Papandreou's failing health. However, the leadership transition from Papandreou to Simitis was confrontational by Papandreou's loyalists, who wanted to prevent such a transition from being realized. Simitis lacked his predecessor's charisma, and the fractured party's support limited many of his actions in government. However, by the end of his tenure in 2004, Simitis had several significant achievements and reforms in the wider society and economy to proclaim. After two decades of the exuberant rhetoric of Simitis' predecessors and financial stagnation, the Greek economy was put in order and became one of the fastest-growing economies in Europe, with an average annual increase of 4.1% of gross domestic product (GDP). The performance of the Greek economy under Simitis sealed the Greek entry into the Euro currency, closing the journey of aligning Greece with the West, which started with Eleftherios Venizelos and continued with Constantine Karamanlis. Simitis also succeeded in the Cypriot accession into the EU, a diplomatic priority for Greece. The successful completion of the Athens 2004 Summer Olympics also boosted Greece's positive image as a modern state capable of undertaking sophisticated tasks. Moreover, a variety of large-scale infrastructure projects were completed, like the new Eleftherios Venizelos airport, the Athens Ring road, Athens Metro, and Rio–Antirrio Bridge. New institutions were also introduced, such as the Greek Ombudsman and several regulatory bodies to supervise market liberalization.

After the debt crisis erupted in Greece in 2009, the legacy of Simitis was re-interpreted by critics as insufficient or misleading. However, government institutions under Simitis were developed and strengthened in their capacity to reform, appearing both more 'modern' and 'European.'

Costas Simitis was born in Piraeus to Georgios Simitis, a Professor at the School of Economic and Commercial Sciences who later became a member of the National Liberation Front government in World War II, and to his wife Fani (née Christopoulou). He studied law at the University of Marburg in Germany and economics at the London School of Economics. He was married to Daphne Arkadiou (b. 1938) and had two daughters, Fiona and Marilena. He resided in the Kolonaki district of Athens. His brother Spiros Simitis was a prominent jurist specializing in data privacy in Germany.

In 1965, he returned to Greece and was one of the founders of the "Alexandros Papanastasiou" political research group. In 1967, after the military coup on 21 April, this group was transformed into Democratic Defense, an organization opposed to the military regime. Simitis fled abroad after planting bombs in the streets of Athens (in later years, he acknowledged his activities on the Greek MEGA TV channel) in order to avoid being jailed and became a member of the Panhellenic Liberation Movement (PAK), led by Andreas Papandreou. He also took up a position as a university lecturer in Germany. He returned to Athens in 1974 and was one of the co-founders of PAK's successor, the Panhellenic Socialist Movement (PASOK). In 1977, he took up a lecturer's post at the Panteion University.

Simitis was not a candidate for the Greek Parliament in the 1981 elections, but he was appointed Minister of Agriculture in the first PASOK government of that year. Following the 1985 elections and his election as a deputy to the Parliament, he became Minister of National Economy; he undertook an unpopular stabilization program, trying to curb inflation and reduce deficits, but resigned his post in 1987 because he felt that his policies were being undermined. In 1993, he took over the Ministry of Commerce and Industry, but in 1995, he again resigned from the ministry and the party's Executive Bureau following a public rebuke he received from Prime Minister Andreas Papandreou.

On 16 January 1996, Papandreou resigned as Prime Minister due to ill health. In a special election held by the party's parliamentary group on 18 January, Simitis was elected in his place over the candidacies of Akis Tsochatzopoulos, Gerasimos Arsenis and Ioannis Charalambopoulos. Papandreou, however, remained Chairman of the party for the next months until his death on 23 June (also Simitis' 60th birthday), just before a party conference would select the party's vice-president; after Papandreou's death, the conference would elect the new Party President. Simitis was elected in PASOK's Fourth Congress on 30 June, defeating Akis Tsochatzopoulos on a platform of support for the European Union.