French construction-materials company Cie. de Saint-Gobain SA, is finding it harder to take its money out of China.

The conglomerate—like all multinationals operating there—faces new delays in recent weeks as Chinese regulators impose tougher restrictions on the movement of capital out of the country to slow the yuan’s decline.

“The process of authorization is going to become longer now,” said Javier Gimeno, who heads Saint-Gobain’s China operations. “The procedures will be controlled more strictly.”

Nearly 7% of Saint-Gobain’s world-wide group sales come from Asia and Oceania, a large part of that from China. The new rules are adding confusion and anxiety to a process that had been getting much easier over the past year, he said. The shift could cause some multinationals to rethink future investments in a country where once-sure payoffs are suddenly facing an uncertain return, analysts say.

As of late November, firms that want to exchange yuan into dollars in China now need approval for any transaction greater than $5 million. They also face tighter limits on amounts they can transfer in and out of bank accounts in China to affiliates in other countries, in a practice known as “cross-border sweeping.”

“We hear a lot questions from corporates about whether they will be able to repatriate their money in the future,” said Alexander Tietze, managing director at Acon Actienbank AG, a German bank that advises companies on Chinese investments. He expects foreign investments in China to slow, and cautioned that foreign takeovers or plans for new joint ventures could fail because of the controls.

With the Chinese economy struggling, multinationals have fewer opportunities to reinvest there, which makes it more difficult for them to do much with money trapped in China.

“A majority of clients are currently consolidating and restructuring their China business,” said Bernd-Uwe Stucken, a lawyer with Pinsent Masons LLP in Shanghai. Some clients are closing down their business, with new investments being the exception to the rule, Mr. Stucken said.

Adding to the confusion: it is unclear where the limits are, because regulators haven’t published official rule changes, but instead have given only informal guidance to banks, according to Daniel Blumen, partner at Treasury Alliance Group, a consulting firm.

Calls to the People’s Bank of China weren’t returned.

One official at a large multinational said that companies can now only sweep amounts out of the country equal to 30% of their Chinese assets. That is down from 100% under previous guidance, but those figures couldn’t be independently confirmed.

One banker at a large European bank said transfers that once took between one to three days can now take more than a week to complete.

The European Chamber of Commerce in China, which represents more than 1,600 European companies, is aware of several cases in which payments by multinational firms are stuck in China awaiting new approvals, said Joerg Wuttke, president of the chamber. The new regulatory scrutiny is disruptive to company operations, and “exacerbates uncertainties regarding the predictability of China’s investment environment,” he said.

China’s State Administration of Foreign Exchange, known as SAFE, and the People’s Bank of China, are reacting to worrisome indicators for investment in the economy. In 2015, foreign firms and individuals poured roughly $250 billion into China, according to the World Bank. While that is almost double the amount in 2009, the recent trends have been negative. Last year marked a second-consecutive foreign-investment decline, and a 14% drop from 2013.

Meanwhile, the yuan has been weakening, and many multinationals have decided they would rather convert their funds to dollars and transfer them out in anticipation of further weakening, said Ker Gibbs, chairman of the American Chamber of Commerce in Shanghai.

China’s currency is down roughly 7% so far this year and the capital flight has diminished the country’s stockpile of foreign currency to $3.05 trillion at the end of November, from $3.23 trillion in January, according to the People’s Bank of China.

The new measures affect small and medium-size firms more severely, as they usually operate with limited amounts of cash. Nevertheless, the new regulation is also hurting bigger firms, said Mikko Huotari, head of the China foreign relations research program at Mercator Institute for China Studies, a German think-tank.

The recent clampdown has whipsawed corporate expectations, said Caroline Owen, founder of RMB Global Advisors, a firm that advises companies on using the yuan.

“China said it is making all these changes to open up the border, but the message companies are now getting is that it’s conditional,” she said “When things get tough, China will just roll things back.”

Write to Vipal Monga at vipal.monga@wsj.com and Nina Trentmann at Nina.Trentmann@wsj.com

Corrections & Amplifications:

Nearly 7% of Saint-Gobain’s world-wide group sales come from Asia and Oceania, a large part of that from China. An earlier version of this article, as well as a photo caption, incorrectly stated 7% of the company’s sales came from China. (Dec. 19, 2016)