Police said their investigation focused on Spark Alliance, a non-profit online platform formed in 2016 that collects donations to provide support to political critics of the city's pro-Beijing authorities.
It is one of two crowd-sourced funding platforms that have collected millions of dollars to provide legal and other help for people arrested in the pro-democracy protests that have upended the city since early June.
But police said some of the donations were allegedly used by the fund owners for other investments.
"We found the donated money was transferred to a shell company and a significant portion of this money was invested in personal insurance products," Senior Superintendent Chan Wai-kei told reporters.
"The beneficiary of these products is the person in charge of the shell company." Four people aged between 17 and 50 -- three men and one woman -- were arrested for money laundering, including the alleged director of the shell company.
Chan did not respond directly to questions from reporters on whether donating to legal defence funds for arrested protesters could count as money laundering.
"Money laundering means you continue to handle the money even when you know it's gained from unlawful activities," he said.
He added people could risk committing offences of inciting or facilitating crimes if a person knowingly financed unlawful activities.
Last month, Spark Alliance announced on its Facebook account that HSBC was suspending its account without explanation.
Neither Spark Alliance nor HSBC responded for requests to comment.
Semi-autonomous Hong Kong has been battered by increasingly violent demonstrations in the starkest challenge the city has presented to Beijing since its 1997 handover from Britain.
Millions have hit the streets in protests fuelled by years of growing fears that authoritarian China is stamping out Hong Kong's liberties.
Police have arrested more than 6,000 people and charged around 1,000 of them, filling the city's courts with cases that are likely to last for years.
Around 40 percent of those arrested are students, some of whom face up to ten years in jail on rioting charges.