President Trump signed an executive order on Monday lifting most of the country’s economic sanctions on Syria, tightening his embrace of a new government in Damascus despite concerns about its leaders’ past ties to Al Qaeda.

The move, which scraped decades of American policy toward Syria, delivered on a surprise announcement by Mr. Trump in May during a trip to the Middle East. At a stop in Saudi Arabia, Mr. Trump met with President Ahmed al-Shara of Syria, who assumed power in December after his fighters deposed the longtime dictator Bashar al-Assad.

Mr. Trump declared Mr. al-Shara, who previously led a rebel group designated by the U.S. government as a terrorist organization, “young, attractive” and “tough,” and said Syria deserved “a chance” to rebuild after a devastating civil war that began in March 2011.

Some current and former U.S. officials remain wary of Mr. al-Shara and his jihadist background, although he cut ties with Al Qaeda several years ago. But Syria’s Arab neighbors are impatient to begin reconstructing Syria, a project that offers not only opportunities to profit but also a chance to stabilize a country that has long exported terrorism, migrants and illegal drugs. Crushing U.S. sanctions imposed during the harsh rule of Mr. Assad, some dating more than 20 years, have kept would-be investors on the sidelines.

Starting Tuesday, Mr. Trump’s executive order dismantles many of those sanctions, including ones against state-linked entities such as Syria’s central bank and other major financial institutions. Sanctions would remain on Mr. al-Assad, who fled to Russia, and his associates, as well as others accused of human rights abuses, drug smuggling and terrorist activities, Karoline Leavitt, the White House press secretary, told reporters on Monday.

Ms. Leavitt said Mr. Trump was delivering on a promise he made in Saudi Arabia that had “shocked the world.”