The king and the president unveiled a commemorative plaque on Monday to mark the event before heading to Damascus for official talks on bilateral and regional issues.
Several cabinet ministers from Syria and Jordan attended the 30-minute ceremony, which came three months after the start of the construction of Al-Wehda dam.
Scheduled for completion by the end of 2005, the dam will provide Jordan with desperately needed water for both human consumption and agriculture, while boosting power supplies to Syria.
Officials said the dam's projected 110 million cubic metre storage capacity would enable 81 million cubic metres of water a year to be supplied to Jordan, a largely desert country which ranks among the world's 10 poorest in water resources.
Around 50 million cubic metres will be used to supply drinking water to the Jordanian capital and the main northern city of Irbid, Jordanian Water Minister Hazim Nasir said.
He warned that the dam "will not solve Jordan's water problems" but said it would make an "important contribution by reducing by around 10% the water deficit in Jordan" which stands at a total of 250 million cubic metres.
An associated hydro-electric project is expected to generate 18,800 megawatt hours of electricity a year, most of which will be supplied to Syria.
Nasir said Jordan would foot most of the bill for building the dam, which he estimated at 66 million dinars ($93 million).
Construction is being funded through loans from the Arab Fund for Economic Development and the Abu Dhabi Development Fund and carried out by Turkish contractor Ozaltin.
An agreement on the dam was signed in April 2003 but plans to build it were already agreed in 1986 and negotiations date back to 1953. The repeated delays were blamed on lack of funds.
King Abd Allah and al-Asad had been due to lay the cornerstone in December but that meeting was postponed for "technical reasons" that were never explained.
The two Arab leaders did not making any statements during the red-carpet ceremony.