The victim was approached by two boys on West Street near the Halifax bank between 11.15am and 11.30am on Thursday.
They followed him down an alleyway towards Havant Bus Station where they threatened him and demanded money.
The suspected teens then followed him back towards the Halifax branch and attempted to obtain more money before running away.
The man was not hurt during the incident, and the two boys stole a small quantity of cash and tobacco.
A spokesman from Hampshire Constabulary said: ‘We would like to speak to anyone who was in the area at the time and may have seen or heard anything.
‘We’d particularly like to speak to two boys who were seen in the area at the time.
‘The first boy is described as aged between 15 and 17 years old, around 5ft, white, larger build, freckles on his nose and upper cheeks, wearing a grey hooded top with an emblem on the front.
‘The second boy is described as aged between 15 and 17-years-old, shorter than 5ft, white, of slim build, wearing a black hoody.’
Anyone with information is asked to call 101 quoting reference number 44210152345, or call Crimestoppers anonymously on 0800 555 111.
Early versions of this story reported that the incident happened at 11.15pm, not 11.15am. Police corrected the initial details that they released.