The candidate will be chosen by a committee of mostly pro-Beijing electors, rather than a public vote, which pro-democracy campaigners have been fighting for.
Carrie Lam, deputy to the current leader, is China's choice for the role and is expected to win.
Ahead of the vote, around 1,000 people took to the streets to demand a greater say in who will lead Hong Kong.
Mrs Lam's main rival, former finance chief John Tsang, is the public's favourite, according to opinion polls.
The third candidate is retired judge Woo Kwok-hing.