British citizens are no longer being advised to avoid all but essential travel to Iran.
UK Foreign Secretary Philip Hammond said there had been “decreasing hostility under President [Hassan] Rouhani’s government”.
The advice brings the UK into line with most of its international partners, officials said.
The move comes weeks after talks ended in agreement on the future shape of Iran’s nuclear programme.
Under the deal, Iran must curb its nuclear activities in exchange for sanctions relief.
Mr Hammond said: “Our policy is to recommend against travel to an area when we judge that the risk is unacceptably high.
“We consider that continues to be the case for specific areas of Iran, notably along Iran’s borders with Iraq, Afghanistan and Pakistan.
“But we believe that in other areas of Iran the risk to British nationals has changed, in part due to decreasing hostility under President Rouhani’s Government”.
The British Embassy remains closed since 2011 – although the British government revealed plans last year to reopen an embassy in Tehran.
Officials said British nationals travelling to Iran should contact the Swedish embassy or the embassy of another EU member state.