The incident happened in Gordon Street, Kettering at about 1am yesterday morning (Thursday).
A police spokesman said: “The victim was walking along Victoria Street and into Gordon Street where he was approached from behind by two men who made off along Mill Road after the robbery.
“They are described as being in their 20s with local accents.
“They wore dark clothing.”
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Witnesses can contact police on 101 or Crimestoppers anonymously on 0800 555111.