A tanker from Louisiana loaded with U.S. natural gas is en route to Portugal, the first shipment in a trade relationship that could shake up the European market.

The 970-foot long Creole Spirit, carrying liquefied natural gas, is expected to arrive by the end of April, according to shipping data and people familiar with the matter.

In Europe, American gas will add to a swell in supply in a crowded market long dominated by Russia. Analysts predict that the arrival of U.S. gas could trigger a price war, leading to lower prices for consumers that could act as a shot in the arm for the struggling European economy.

“It’s the start of the price war between U.S. LNG and pipeline gas,” said Thierry Bros, an analyst at Société Générale. SCGLY 1.76 %

The U.S. began selling Gulf Coast gas abroad for the first time in February, marking its emergence as a major exporter. After a yearslong effort by Houston-based Cheniere Energy, LNG 0.51 % the first shipment went to Brazil, with subsequent cargoes heading to Asia.

Now, Portugal’s energy company Galp Energia GALP -0.25 % has bought the first European cargo from Cheniere’s Sabine Pass facility, according to a person familiar with the transaction. The ship is estimated to arrive at the Port of Sines in Portugal on April 26, according to data from the port.

Galp and Cheniere representatives didn’t respond to requests for comment.

Europe is expected to be a big market for American gas. The shale-gas boom has transformed the energy landscape in the U.S., and the country is now expected to become a net gas exporter in 2017.

Cheniere has signed long-term contracts with a number of European gas companies, including U.K.’s BG Group, which was acquired earlier this year by Royal Dutch Shell RDS.A 0.40 % PLC, and Spain’s Gas Natural.

In Europe, U.S. suppliers will square off with Russia, which currently supplies about a third of the continent’s gas via pipeline. Germany, for example, gets half of its gas and Italy a third from Russia. Norway, Algeria and the Middle East are other major sources of gas for the continent.

Analysts say that Russia could cut the prices it charges its European customers to try to chase away the new U.S. competitors.

Russian gas giant Gazprom said earlier this year it wasn’t planning a price war. But if U.S. LNG prices did fall, the company “would seek to cut its own costs,” Gazprom’s deputy chairman Alexander Medvedev said in February.

At current prices, U.S. LNG delivered to Europe would cost around $4.30 per million British thermal units, according to price reporting agency Argus Media. Russia sells its gas to Europe for $5.80, on average. However, analysts say that Russia could drastically lower prices in a price war, to below $3.

Pipeline gas can be cheaper than LNG, because that gas has to be liquefied, shipped and re-gasified at arrival. But many in Europe see the U.S. entry into the market as part of a broader geopolitical effort to challenge Russian domination of energy supplies and prices.

The impact of U.S. gas in Europe “will be gradual, but it does start to change everything,” said Trevor Sikorski of London-based consultancy Energy Aspects. “The new LNG will put downward pressure on prices, and losing both volume and value could be a hard pill to swallow” for Russia.

—Chester Dawson contributed to this article.

Write to Georgi Kantchev at georgi.kantchev@wsj.com

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