The Supreme Court sided with American Express on Monday in a closely watched case over credit card fees, with major implications not only for the credit card industry but also for some of the country’s biggest platform companies such as Google and Facebook, analysts said.

In a 5-to−4 decision, the court held that American Express is allowed to use contract language to bar retailers and merchants from steering their customers toward paying with competing credit card networks such as Visa and Mastercard.

American Express charges merchants higher fees to use its payment platform, which it says helps the company deliver better cardholder benefits. But the higher fees have led some merchants to encourage consumers to pay using cheaper, non-Amex cards. To thwart that practice, Amex has sought to use so-called “anti-steering provisions” in its merchant contracts — legal language that the Justice Department and a number of states alleged is anticompetitive and harms merchants.

The court’s ruling Monday found that the plaintiffs — the Justice Department and several states, including Ohio — needed to go further and prove that Amex’s practices were harmful to both merchants and consumers alike.

“Their argument — that Amex’s antisteering provisions increase merchant fees — wrongly focuses on just one side of the market,” wrote Justice Clarence Thomas for the court.

The ruling is a major victory for American Express, which said Monday that the decision “will help to promote competition and innovation in the payments industry.” The Justice Department declined to comment.

The decision in Ohio v. American Express will likely be interpreted to apply to other major platforms, including those in the tech industry, analysts say. Similar to Amex, companies such as Amazon, Facebook and Google operate in what economists call two-sided markets: All serve average Internet users on the one hand, while doing business with third-party companies — including independent retailers and advertisers — on the other. (Amazon chief executive Jeffrey P. Bezos also owns The Washington Post.)

The Court’s ruling could shield tech platforms from litigation if plaintiffs cannot prove that both sides of a two-sided market are being harmed by the platform’s practices, said Hal Singer, an economist at George Washington University’s Institute of Public Policy.

“What this would do is that it would bar an advertiser, or the government on behalf of advertisers, from bringing a case against Google unless it could also show harm to consumers,” said Singer.

The ruling could feed into a wider debate about the power of large tech companies at a time when firms such as Google and Facebook are facing scrutiny for their role in shaping online speech and influencing the outcome of elections.

“For the tech companies, this creates an enormous shield against challenges to their dominant positions as small competitors assert harms and certain segments of consumers assert harms,” said Gene Kimmelman, a former Justice Department antitrust official who now heads the consumer advocacy group Public Knowledge.

Tech industry advocates had argued before the court that judges should consider both sides of a two-sided market in evaluating whether a company is behaving anticompetitively.

“Conduct that might appear anticompetitive when only one set of customers is considered may in fact be entirely consistent with — and actually promote — healthy competition when competition on both sides is considered,” wrote the Computer and Communications Industry Association in a court brief.

Stephanie Martz, senior vice president and general counsel for the National Retail Federation, said the group disagrees with the ruling because it bars merchants from being transparent about swipe fees, which are ultimately passed on to consumers in the form of higher prices. “We think that the court got this pretty dramatically wrong,” Martz said, “because there’s no question that when a consumer has more information rather than less information she makes better decisions.”

Martz said the benefits credit cards offer to consumers should be treated separately from the fees they charge merchants. Instead, the current system allows card companies to “subsidize” the amenities they offer to cardholders with the fees they charge businesses, Martz said.

Indeed, some credit card experts say that a ruling against American Express could have taken the luster out of some credit cards rewards programs by making it more expensive for card companies to woo customers by offering flights, cashback and other perks.

Amex relies more heavily on swipe fees than other credit card issuers since less of its revenue comes from interest rate charges, said Matt Schulz, a senior industry analyst for Many of Amex’s customers are high earners who pay their balances off completely each month, which prevents them from paying interest on purchases. The tilt toward transaction fees can also be traced back to the first American Express credit card, which was a “charge card” that needed to be paid back in full each month, Shulz said.

Other experts said that consumers are unlikely to experience rising prices as a result of the ruling, because it simply upholds the existing relationship between Amex and merchants.

“American Express has been around for a long time,” said Jeffrey Blumenfeld, an antitrust attorney at Lowenstein Sandler in Washington. “I don’t know that we’ve seen any negative price effects from its existence, and I don’t see why this decision would change merchants’ pricing when it doesn’t change how Amex interacts with merchants.”