Hard seltzer, the summer drink of the pandemic, has lost its cool.

Sales of the low-calorie, fruity alternative to beer have plunged this year, marking a sharp reversal for a once-hot market that attracted entrants from Bud Light, Corona and others. Hard seltzer sales in retail stores, which rose about 11% in 2021, fell about 18% year-over-year in the four weeks ended July 2, according to an analysis of Nielsen data by Bump Williams Company.

“Hard seltzer’s lost its novelty as consumers have been distracted by many new beyond-beer products entering a hyper-crowded marketplace,” said Dave Burwick, chief executive of Boston Beer Co. , which produces Truly hard seltzer, on a call with analysts Thursday.

Consumers, especially 35- to 44-year-olds, are shifting back to light beers, which have lower prices than hard seltzers, Mr. Burwick said. The Truly brand, whose sales fell 17% in the second quarter from a year ago, is also losing ground to rivals.

Thursday’s call isn’t the first time Boston Beer executives have flashed warning signs of a changing market outlook. The company said it had overestimated demand for Truly, the No. 2 brand behind White Claw, last summer and that the hard seltzer outlook for the rest of the year was uncertain.

Hard seltzers emerged several years ago as one of the hottest parts of the alcohol market, tempting health-conscious drinkers away from beer. In response, more beer brands jumped into the arena. Natural Light’s seltzer launched in summer of 2019. Michelob Ultra joined in. More recently, soda giants such as Coca-Cola Co. and liquor companies such as Bacardi Ltd. have rolled out canned cocktails that compete for shelf space.

Riding the wave, many of these companies built out their offerings beyond seltzers to include other canned drinks with flavors like cola, lemonade, tea and margarita.

The changing seltzer sentiment comes in contrast to trends seen during the pandemic, when Nielsen IQ in 2020 said hard seltzers had the “most sustainable growth trajectory across the U.S. alcohol landscape.”

Boston Beer, which also brews Samuel Adams, lowered its earnings targets for the year following its weak quarterly results. The company now expects hard seltzer volumes to fall between 15% and 20% this year, after earlier projecting volumes would be flat to down 10%.

Write to Alex Harring at alex.harring@wsj.com