The government "will take steps to make it clear that foreign entities should not fund, support or influence such events held at the Speakers' Corner", the interior ministry said in statement, referring to Singapore's only legal protest area.
The warning against foreign involvement with the annual Pink Dot rally comes as Singapore's government is taking an increasingly conservative stance against gay rights.
The ministry said it had released the statement on Sunday in response to media queries about whether foreign companies can legally provide sponsorship for the event, which was held this year on June 4 and also sponsored by Twitter, BP and major banks Barclays and JP Morgan.
"These are political, social or moral choices for Singaporeans to decide for ourselves. LGBT issues are one such example," it added.
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In a separate statement released today, the ministry said that no action will be taken against foreign companies that sponsored the event this year.
Pink Dot's organisers defended their choice of sponsors, saying they had done all they could to comply with Singapore's laws.
"Our Corporate Sponsors that have supported us over the years are all registered and incorporated in Singapore," Pink Dot said in a statement posted on its Facebook page.
Started in 2009, the rally has grown despite a backlash from social and religious conservatives in the multi-racial nation of 5.5 million.
More than 28,000 attended last year's event but organisers said they did not keep track of attendance this year.
Gay rights movements in the wealthy city-state have grown steadily in recent years, helped by changing social norms among the younger generation and a large influx of tourists and expatriates.
In an interview with journalists last year, Singapore prime minister Lee Hsien Loong said the country was not ready to legalise same-sex marriage, but added that the local gay community is not harassed or discriminated against.