Police said Thomas Griffiths, of no fixed address was spotted acting suspiciously by officers in The Forum on November 14.
The 21-year-old attempted to run from the scene but was detained by police after a short pursuit.
Police said a rucksack he was carrying was searched and a large quantity of cannabis and cash was found inside.
He was arrested and charged with possession with intent to supply class B drugs (cannabis), possession of a bladed article in a public place and the acquire, use or possession of criminal property.
Police said Griffiths pleaded guilty and appeared at Lewes Crown Court on Friday (February 1) for sentencing.
He was given an 18 months in prison sentence, as well as an order for forfeiture of the drugs and cash and was ordered to pay a £140 victim surcharge.
Investigating officer Tom Hawkins said: “Our officers had attended the area for a different matter when they spotted Griffiths acting suspiciously and dropping the rucksack he was carrying.
“It is vital that we do all we can to prevent the supply of drugs causing harm to the community.
“We prioritise crimes that cause the greatest harm to victims and communities and encourage the public to report drugs issues to us without delay.”