RIYADH, May 20 (Reuters) - Saudi Arabia's Public Investment Fund, its top sovereign wealth fund, will ramp up its overseas investment only slowly, the fund's managing director Yasir al-Rumayyan said on Saturday.

The fund, which is believed to have assets totalling about $183 billion, currently has more than 90 percent of its portfolio within Saudi Arabia, largely in the form of stakes in state-affiliated companies.

But under economic reforms launched last year, it plans to invest more abroad to obtain technology for Saudi industry and boost returns on Saudi Arabia's financial reserves as the kingdom tries to diversify its economy beyond oil.

"The shift to overseas investment must be gradual," Rumayyan told a conference of senior U.S. and Saudi business executives, coinciding with the visit of U.S. President Donald Trump to Riyadh.

Abroad, the PIF will put money into a range of international investment pools and fixed income, while domestically it will invest in projects that develop industries in line with the Saudi economic reforms, Rumayyan added.

(Reporting by Celine Aswad and Katie Paul; Writing by Andrew Torchia) ((andrew.torchia@thomsonreuters.com; +9715 6681 7277; Reuters Messaging: andrew.torchia.thomsonreuters.com@reuters.net))