The city has the cheapest ecstasy, the second-cheapest cannabis and is also one of the cheapest places for heroin.
Drug prices are so low it has attracted addicts from elsewhere in the country to move to the city to get cheap fixes.
The survey, by Druglink magazine, published by the charity DrugScope, showed while the national average for ecstasy is £3 a pill, in Portsmouth the price is just 50p.
Last year Portsmouth was the most expensive city in the UK for cannabis resin – but prices have halved, making it the second-cheapest.
Prices for a gramme of heroin have also dramatically fallen, from up to £90 in 2004 to around £40 today. Local drugs workers claim the fall in heroin prices is the direct result of a group of dealers targeting rehab centres by distributing business cards with price lists offering cut-price bags of heroin.
Users from as far away as Bristol and Birmingham, many of them homeless, have resettled in Portsmouth in order to be near the cheap drugs.
The dealers are thought to have left the city last month, but have left many users with raging habits.