"It was a great surprise and I obviously didn't expect it," Halzen said, speaking to the committee by phone from Italy, in a call broadcast at the news conference to announce the winner on Tuesday.
Halzen said it was predicted before that he would win the Nobel Prize but the announcement still made him feel "strange".
"I'm working on a proposal and I hope winning this will help getting it approved," he said to chuckles.
The Nobel Committee for Physics also cited Halzen's work at the IceCube Neutrino Observatory in Antarctica, where he began researching the capture of neutrinos in the 1980s.
"Francis Halzen has led an international team of researchers and engineers who have provided us with a fantastic instrument. His tenacity and scientific vision has paved the way for a new kind of astronomy," Mark Pearce, chair of the Nobel Committee for Physics, said in a news release.
Born in Belgium, the 82-year-old scientist is affiliated with the University of Wisconsin-Madison in the US, the lead institution at the IceCube Neutrino Observatory.
Neutrinos are tiny cosmic particles with a mind-bogglingly small mass.
Yet they are everywhere: they spew from stars such as the sun and trillions zip through our bodies every second.
Scientists cannot glimpse the mysterious ghost particles zooming around on their own.
Instead, they measure what happens when the tiny particles collide with other bits of matter, producing flashes of light or charged particles.
Halzen studied a rare gang of neutrinos that scientists consider messengers from the cosmos, offering clues to how the universe evolved.
A representative from the IceCube Neutrino Observatory did not immediately return a request for comment.
The University of Wisconsin-Madison did not immediately have a comment when reached by the AP.
The Royal Swedish Academy of Sciences announced the award in Stockholm.
The prize money this year is 12 million Swedish kronor ($A1.7 million), whether for an individual, a small group of laureates or an organisation.
In 2025, three scientists won the Nobel Prize in physics for research on the strange behaviour of subatomic particles called quantum tunnelling that enabled the ultra-sensitive measurements achieved by MRI machines and laid the groundwork for better mobile phones and faster computers.
The announcements of 2026's prizes began Monday with the Nobel Prize in medicine being awarded to three scientists whose work led to a tool that uses light to help unravel how the brain works.
This week's awards continue on Wednesday with the chemistry prize and literature on Thursday.
After Friday's peace prize is awarded, the Nobel Memorial Prize in Economic Sciences will be announced on Monday.
with Reuters
