The Nabucco pipeline (also referred as Turkey–Austria gas pipeline) was a failed natural gas pipeline project from Erzurum, Turkey, to Baumgarten an der March, Austria to diversify natural gas suppliers and delivery routes for Europe. The pipeline was to lessen European dependence on Russian energy. The project was backed by several European Union states and the United States and was seen as a rival to the Gazprom-Eni South Stream pipeline project. The main supplier was to be Iraq with potential supplies from Azerbaijan, Turkmenistan, and Egypt.

The project was developed by a consortium of six companies. Preparations started in 2002 and the intergovernmental agreement among Turkey, Romania, Bulgaria, Hungary and Austria was signed on 13 July 2009. After an announcement of the construction of TANAP, the consortium submitted the Nabucco-West project, which was to run from the Turkish-Bulgarian border to Austria. It was a modification of the original Nabucco Pipeline project. The main supply for Nabucco-West was to be the Shah Deniz gas through the now operational Trans-Anatolian Pipeline (TANAP). After the Shah Deniz consortium decided to prefer the Trans-Adriatic Pipeline over Nabucco, the Nabucco pipeline plan was finally aborted in June 2013.

The Nabucco project was backed by the European Union and the United States. In the Trans-European Networks – Energy programme, the Nabucco pipeline is designated as a project of strategic importance. An objective of the project is to connect the European Union better to the natural gas sources in the Caspian Sea and the Middle East regions. The project has been driven by the intention to diversify its current energy supplies, and to lessen European dependence on Russian energy—the biggest supplier of gas to Europe. The Russia–Ukraine gas disputes have been one of the factors driving the search for alternative suppliers, sources, and routes. Moreover, as per the European Commission, Europe's gas consumption is expected to increase from 502 billion cubic meters, in 2005, to 815 billion cubic meters in 2030, which would mean Russia alone would not be able to meet the demand.

South Eastern Europe is important as many of the regions are heavily dependent on Russian gas imports. Nabucco aims to diversify the gas supply to increase competition and security. Simon Pirani, senior research fellow, Oxford Institute for Energy Studies presented to delegates at the Ukrainian Energy Forum in 2013 a list of prices from the Russian newspaper Izvestia: "What they show is the prices at which Russian gas is being purchased in different European countries, and this tells quite a simple story. If you're in Eastern Europe, and you are quite heavily dependent on Russian gas, you pay more than $500/TCM; if you're in the UK, where we have a pretty much complete domination of gas-to-gas market, you pay $300, or $370+ in Germany, which is somewhere in between."

Preparations for the Nabucco project started in February 2002 when first talks took place between Austrian OMV and Turkish BOTAŞ. In June 2002, five companies (OMV of Austria, MOL Group of Hungary, Bulgargaz of Bulgaria, Transgaz of Romania and BOTAŞ of Turkey) signed a protocol of intention to construct the Nabucco pipeline. The protocol followed by the cooperation agreement in October 2002. The name Nabucco comes from the same famous opera of Giuseppe Verdi, which was given to the project in the very early phases by Lutfu Atasoy (a consultant to Erdemir Engineering Company in connection with BOTAŞ) in a request for inquiry for pipeline engineering services to Kent plc. Atasoy wanted to shorten the original verbose project name, and decided to suggest Nabucco, which he had been listening to throughout his car ride on the way to the meeting with the General Manager of Erdemir Engineering Company at the time. This arbitrary suggestion has interestingly gained lots of popularity since, likely causing the five partners to attend a live premiere of the opera at the Vienna State Opera. In December 2003, the European Commission awarded a grant in the amount of 50% of the estimated total eligible cost of the feasibility study including market analysis, and technical, economic and financial studies. On 28 June 2005, the joint venture agreement was signed by five Nabucco partners. The ministerial statement on the Nabucco pipeline was signed on 26 June 2006 in Vienna. On 12 September 2007, Jozias van Aartsen was nominated by the European Commission as the Nabucco project coordinator. In February 2008, German RWE became a shareholder of the consortium.

On 11 June 2008, the first contract to supply gas from Azerbaijan through the Nabucco pipeline to Bulgaria was signed. The President of Azerbaijan Ilham Aliyev confirmed on 29 January 2009, that Azerbaijan was planning to at least double its gas production in the coming five years to supply the pipeline. On 12 April 2009, the Minister of Energy of Turkey Hilmi Güler confirmed that Turkey is ready to sign a deal, provided that Turkey gets 15% of the natural gas to be carried through the Nabucco pipeline.

On 27 January 2009, the Nabucco Summit held in Budapest. On 24–25 April 2009, the Nabucco pipeline was discussed, among other energy issues, at the high-level energy summit in Sofia, and on 8 May 2009, at the Southern Corridor Summit in Prague.

The intergovernmental agreement between Turkey, Romania, Bulgaria, Hungary and Austria was signed by five prime ministers on 13 July 2009 in Ankara. The European Union was represented at the ceremony by the President Jose Manuel Barroso and the Commissioner for Energy Andris Piebalgs, and the United States was represented by Special Envoy for Eurasian Energy Richard Morningstar and Ranking Member of the United States Senate Committee on Foreign Relations Senator Richard Lugar. Hungary ratified the agreement on 20 October 2009. Bulgaria ratified the agreement on 3 February 2010. Romania ratified the agreement on 16 February 2010. Turkey became the final country ratifying the agreement on 4 March 2010.