DUBAI, May 18 (Reuters) - Stock markets in the Gulf may follow global shares lower on Thursday as sentiment has soured because of rising uncertainty over the future of U.S. President Donald Trump.

MSCI's broadest index of Asia-Pacific shares outside Japan dropped 0.4 percent and Japan's Nikkei shed 1.5 percent, taking cues from the largest single-day decline overnight on Wall Street since Trump was elected president in November. The S&P 500 dropped almost 1.8 percent.

Exchanges in the region most sensitive to foreign fund flows, such as Dubai and Qatar, may be the hardest hit on Thursday as institutional managers unwind positions and sit out of markets until there is more clarity.

Meanwhile, Trump's trip to Saudi Arabia in the coming days, may not be a market trigger, many traders told Reuters, although they are reserving some optimism that deeper ties with U.S. companies could be beneficial for some companies. Most are expecting mega deals from the energy and defense sectors.

Saudi Arabia's index has so far been outperforming its regional peers as investors build positions ahead of the holy month of Ramadan, that traditionally sees very low volumes.

(Reporting by Celine Aswad; Editing by Sunil Nair) ((celine.aswad@thomsonreuters.com)(+9715 62247653)(Reuters Messaging: celine.aswad.thomsonreuters.com@reuters.net))