Former Nato Secretary General Javier Solana has been denied an automatic visa waiver for entry to the US.
His application for an Electronic System for Travel Authorization (Esta) was denied because of a visit to Iran.
The Spanish citizen must instead apply for a full visa - a more expensive and lengthy process.
Mr Solana, 75, headed Nato between 1995 and 1999. He later became EU foreign policy chief, and helped negotiate the 2015 nuclear deal with Iran.
The visa rules date from the Obama administration, and are unconnected to President Donald Trump's controversial travel ban, which targets Iran and five other Muslim-majority countries.
They must instead apply for a visa, which involves a face-to-face interview at a US embassy or consulate.
Mr Solana last went to Iran in 2013 when he attended President Hassan Rouhani's inauguration.
"I went there to represent all those who had been involved in the negotiations," he told Spanish TV. "It's a bit of a mean decision."
President Trump announced the US withdrawal from the Iran nuclear deal in May.
The former Nato chief blamed "an algorithm" for the denied application, and said he needed entry to the US because he teaches at various universities.
British-Iranian BBC journalist Rana Rahimpour was denied entry to the US in January 2016, despite her dual nationality.