India’s richest man, Mukesh Ambani, took the first step toward putting in place a succession plan for his family-run conglomerate Reliance Industries Ltd. , handing over the reins of a prominent group company to his eldest son.

Akash Ambani, 30, was elevated from a director to chairman of the board of Reliance Jio Infocomm Ltd., India’s largest wireless-network provider, according to a regulatory filing on Tuesday, while Mukesh Ambani resigned from the board.

The step provides welcome certainty for India’s most valuable company, with Mukesh Ambani having turned 65 earlier this year. Many of India’s biggest companies are family-run enterprises, and in some cases, the lack of a clear succession plan has led to long-running disputes—most famously at Reliance itself.

After founder Dhirubhai Ambani died in 2002 aged 69, Mukesh Ambani and his younger brother, Anil Ambani, were enmeshed in a yearslong battle over the control of Reliance units. Ultimately, the elder Ambani brother prevailed.

“There is a clear message conveyed to the investors that there is a succession plan in the offing,” Harshvardhan Dole, vice president of research at a unit of brokerage firm IIFL Securities Ltd. , said of the move.

Shares of Reliance Industries rose nearly 2% on Wednesday, even though the broader S&P BSE Sensex index slipped around 0.3%.

Akash Ambani, in a red tie, with his father, Mukesh Ambani.

Photo: Shailesh Andrade/REUTERS

With a market capitalization of around $220 billion, Reliance is a giant in India’s economy. Mr. Ambani’s wealth is estimated to be around $90 billion, according to Forbes.

Though the mainstay of Reliance has long been its oil and petrochemicals business—it runs the world’s largest oil refinery in the western state of Gujarat—in recent years, the company has diversified into such businesses as telecom and retail. It has also announced plans to develop its renewables business.

The expansion is part of Mr. Ambani’s vision to pivot the group toward digital commerce as a legacy for his children, according to people familiar with his thinking.

The launch of Jio’s wireless network in 2016, which offered data for as low as 75 cents for a gigabyte, changed the landscape of India’s telecom industry and brought cheap internet to millions of people across the country. It soon grew to become India’s largest mobile-network provider, with more than 400 million subscribers recently.

In 2020, major U.S. companies, including Facebook Inc., Alphabet Inc.’s Google and private-equity firms like General Atlantic and Silver Lake invested a total of more than $13 billion into the mobile operator’s parent company Jio Platforms Ltd. Last year, Jio and Google launched what they say is the world’s most affordable smartphone, which costs around $50 in India.

Akash Ambani and his twin sister, Isha Ambani,

have been on the board of Jio Infocomm and the group’s consumer-facing unit, Reliance Retail Ventures Ltd., since at least 2014. Both of them were involved in the discussions for Facebook’s investment, according to people familiar with the talks. Mukesh Ambani’s younger son, Anant Ambani, serves on the board of Jio Platforms.

Isha Ambani has been playing an active role at Reliance Retail, which includes companies that sell groceries, fashion wear and the world’s largest toy retailer Hamleys. Akash Ambani’s elevation raises expectations that a similar step could be in the offing for her as well.

“Both children have been groomed within the group for the last few years,” said Mr. Dole.

Write to Shefali Anand at shefali.anand@wsj.com