MEXICO CITY—Mexico has significantly reduced imports of U.S. gasoline since President Andrés Manuel López Obrador took office Dec. 1, as a government crackdown on fuel theft has jammed up distribution and caused widespread gasoline shortages.

Seaborne gasoline imports from the U.S. Gulf Coast have averaged about 364,000 barrels a day under Mr. López Obrador, a 25% decline from December 2017 and January 2018, according to research firm ClipperData.

The decline is the result of fewer orders of American gasoline and congestion at Mexico’s Gulf Coast fuel terminals where at least 11 tanker vessels were idling in the Gulf of Mexico waiting to unload about 2.5 million barrels of gasoline as of Sunday afternoon, ClipperData said.

It also underscores the depth of a worsening fuel shortage across much of the country that is angering motorists by causing hourslong queues for gasoline, prompting some public transport to shut down, and causing losses to companies across central Mexico.

The government has explained the shortages as the result of Mr. López Obrador’s decision to order the shutdown of several key pipelines that transport gasoline from refineries and coastal terminals to Mexico’s biggest cities to combat rampant fuel theft which the government says cost state oil firm Petróleos Mexicanos, or Pemex, roughly $3 billion last year.