Li Ka-shing's announcement came shortly before the city's chief executive Carrie Lam announced a law banning face masks in public, the latest move from a government that has struggled to control four months of increasingly violent pro-democracy demonstrations.
The 91-year-old billionaire said small and medium-sized businesses would benefit from the fund, worth HK$1 billion (US$128 million), which will be distributed in partnership with the government.
It comes as Hong Kong's economy faces "unprecedented challenges", Li's foundation said in a statement.
This summer's protests, sparked by rising public anger towards Beijing's rule, has battered the economy which was already struggling with the US-China trade war fallout.