Sports stars, teams and leagues have long used Instagram to promote themselves and build relationships with fans. Increasingly, they’re using it to sell products, too.

On Nov. 20, tennis star Serena Williams and her S by Serena fashion brand released a new sequin jacket that could only be purchased through Instagram’s “Checkout” feature. (A jumpsuit was also made available on Instagram, but people could also buy that on SerenaWilliams.com.)

Instagram drove more than 50% of sales for the S by Serena brand in the week following the jacket’s introduction, said Stephanie Lim, head of e-commerce for SerenaWilliams.com.

The platform is already the primary venue for social merchandising efforts by the NBA’s Los Angeles Clippers, said Alessandro Gasparro, director of social media for the team. This summer the Clippers used “shoppable” posts on Instagram, which drive users back to the team’s online store, to sell jerseys for new players Kawhi Leonard and Paul George, as well as Buffalo Braves-branded throwback uniforms.

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Instagram is making commerce a major priority as consumers increasingly flock to mobile devices to shop. Smartphones accounted for 35.6% of online sales from Nov. 1 to Dec. 2, up from 28.8% last year, according to Adobe Analytics.

And more than 58% of visits to retail sites were driven by smartphones in that period, up 16%, Adobe said. Online sales totaled $81.5 billion from Nov. 1 to Dec. 2.

But social networks themselves aren’t yet meaningful drivers of commerce.

On Cyber Monday, social networks delivered just 6.7% of visits to e-commerce sites, up 15.1% year over year, according to Adobe. The users they brought comprised just 2.1% of all sales, Adobe said.

Nor is Instagram always the strongest performer. Twitter Inc. was responsible for roughly 75% of social revenue for the Clippers’ Buffalo Braves throwbacks, Mr. Gasparro said.

The platform shows promise as a sales driver, particularly as Instagram invests more in features meant to help, said another sports media executive who has tested Instagram commerce, speaking on condition of anonymity. But early tests drove very little in actual sales, he said.

Making plays

Instagram has been encouraging the experiments from the sports world, said Dev Sethi, head of sports for Instagram. The company, part of Facebook Inc., sees value in sports leagues, teams and athletes as their own type of fashion influencers. Athletes such as Ms. Williams, Tom Brady and countless NBA superstars have personal fashion brands they’re looking to promote, Mr. Sethi said.

“Shopping as a feature is a main priority for Instagram Sports,” Mr. Sethi said.

There are multiple ways for athletes and businesses to sell products through Instagram. “Shopping tags,” which rolled out in November 2016 in the U.S. and are now available globally, direct users to the seller’s website.

More than 130 million people tap to reveal product tags in shopping posts every month, an Instagram spokesman said.

Instagram’s Checkout feature, which launched in the U.S. in March, lets users complete purchases without leaving the app.

It isn’t just for brands to use: In the spring, Instagram kicked off a program that lets creators—media companies such as Refinery29, models such as Gigi Hadid and athletes, among others—tag a brand’s products and let users buy them using Checkout. James Harden of the Houston Rockets used it to offer his Adidas shoes for sales via a post in his own feed, which has nearly 11 million followers.

Instagram doesn’t collect revenue when sales take place on other companies’ websites, but charges a processing fee for Checkout sales directly on its platform, Mr. Sethi said. He declined to be more specific or to describe the revenue Checkout is generating for Instagram or its commerce partners.

“It’s a selling fee that helps us fund programs and product development and offset credit-card processing costs,” Mr. Sethi said.

With Checkout, Instagram shares customers’ delivery information and will provide a customer’s email if they have opted in to share that information with sellers, a spokesman said. It doesn’t share credit card or other payment information.

The platform is also testing a function that lets brands run their shoppable posts as ads, though Checkout is not part of those tests, a spokesman said.

Ms. Lim, the e-commerce chief at SerenaWilliams.com, cautioned that brands shouldn’t become too dependent on Instagram commerce if it does take off.

Instagram can become an important part of what is ideally a diverse portfolio of sales channels if it brings in new customers and sales that a brand wouldn’t have gotten on its own, Ms. Lim said. There is still value in directing users to brands’ own websites, however, where brands can have greater control over the shopping experience, she said.

“In terms of what acquisition channels you rely on, I would always advise against putting all of your eggs in one basket,” she said.