BOGOR, Indonesia—King Salman of Saudi Arabia began a 12-day visit to Indonesia Wednesday meant to cement plans for a $6 billion refinery project and build up ties between the two Muslim-majority countries, with Jakarta eyeing new funding for an underdeveloped petroleum industry.

King Salman, in the midst of a month-long tour through Asia to deepen ties in the region and hedge against an unpredictable U.S. government, arrived in Southeast Asia’s largest economy just months after his kingdom signed a $6 billion deal to upgrade a refinery on Indonesia’s main island. On his first stop earlier this week, he agreed to invest  $7 billion in a petrochemical project in nearby Malaysia.

Indonesian President Joko Widodo, who has been pushing for more foreign investment in infrastructure in his resource-rich country, views the trip in part as an opportunity to draw Saudi investment to oil and other projects on some of Indonesia’s largest islands, including Sumatra and Borneo, Indonesia’s foreign minister told reporters.

The visit marks “a turning point” for two nations “united by Islam, brotherhood and a mutually beneficial relationship,” Mr. Widodo said in welcoming King Salman. Ministers from both countries signed preliminary deals in multiple fields, including education, fisheries, cultural exchange, health and development financing, without providing details.

The visit to Indonesia is the first by a Saudi king in almost 50 years. On Wednesday, Mr. Widodo and King Salman proceeded from a military air base to a presidential palace outside Jakarta under a pouring tropical rain. Authorities in the region of more than 30 million people temporarily closed a major toll road to provide passage to an entourage of hundreds of princes, ministers and businessmen.

For Saudi Arabia, Indonesia’s pressing needs for infrastructure offer the king options to diversify the kingdom’s overseas investments away from oil, said Yon Machmudi, a Middle East expert at the University of Indonesia.

“The relation between the two countries has been relatively stagnant,” he said, saying the trip appeared aimed at turning that around.

Indonesia is the region’s largest producer of oil but has long struggled to attract the investment it needs to refine more of it domestically. Indonesia’s last new refinery was built in the early 1990s, and capacity hasn’t increased in more than 20 years.

The $6 billion refinery project, a Pertamina-controlled refinery upgrade in which Saudi Aramco will take a 45% stake, is located on Indonesia’s main island of Java. The two state-owned companies signed the deal late last year.

King Salman’s monthlong trip to Asia includes building ties with predominantly Muslim countries such as Malaysia and Indonesia, the world’s most-populous Muslim-majority nation, as well as Brunei and the Maldives. The king is scheduled to speak Thursday to the country’s parliament and to visit Jakarta’s Istiqlal mosque, the nation’s largest, and meet religious leaders.

Later this week, the entourage travels to the resort island of Bali, several hours east of the capital, for a weeklong stay.

Saudi Arabia is also seeking investment and stronger ties with economic powerhouses Japan and China. The trip is part of a plan to diversify Saudi Arabia’s pool of allies as ties with the U.S. have been tested by the Obama administration’s outreach to Iran, the kingdom’s rival.

—I Made Sentana contributed to this article.