A declaration was signed in Ankara during celebrations to mark the 75th anniversary of the Turkish republic by the presidents of Turkey, Azerbaijan, Georgia, Kazakhstan and Uzbekistan.
The President of Turkmenistan is reported to have pulled out of the ceremony at the last moment, but it is not known why.
The agreement was witnessed by the US Energy Secretary, Bill Richardson - a sign of Washington's commitment to the pipeline.
The BBC Ankara Correspondent Chris Morris says the agreement sends a strong political message to the mainly western oil companies which are due to make a final recommendation next month on the main export route for Caspian oil.
The Ceyhan pipeline is an expensive option for the oil firms, whose main concern is to export their product as cheaply as possible.
The amount of oil currently being produced in the Caspian does not justify the possible $4bn building costs.
But the companies have been put under considerable political pressure in Washington and elsewhere as the debate has intensified in the last few weeks.
Turkish and American officials in Ankara are convinced that the pipeline will be commercially viable.
Turkey also insists that the alternative routes via Russia or Georgia are unacceptable because it would mean a huge increase in tanker traffic through the Bosphorus, the narrow strait which runs through the heart of Istanbul.
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