Stewart is due to meet political and business leaders, as well as human rights groups and NGOs, the UK foreign ministry said in a statement.
His arrival in the capital Harare comes just days after Mugabe’s resignation on Tuesday, ending his 37-year rule as MPs gathered in parliament to impeach their 93-year-old leader.
Stewart described the historic change as “an absolutely critical moment” following “Mugabe’s ruinous rule”.
“The events of the last few days have given people here real hope that Zimbabwe can be set on a different, more democratic and more prosperous path,” he said in a statement.
Britain is Zimbabwe’s former colonial ruler.
Around 20,000 British citizens live in Zimbabwe and some 112,000 Zimbabweans live in Britain.
Britain provides around £86 million (97 million euros, $114 million) in aid to Zimbabwe every year.
But relations were rocky under Mugabe, particularly over the expropriation of white-owned farms there.
“What comes next must be driven by Zimbabweans — it must be in line with the Zimbabwean constitution and will be impossible without clear resolve from the incoming government. That is what my visit here is all about.
“Britain wants to be a genuine partner for Zimbabweans as they forge a new future,” Stewart said.
Zimbabwe’s incoming president Emmerson Mnangagwa is due to be sworn in on Friday, following a triumphant return home this week after he was sacked as Mugabe’s deputy earlier this month.