The £35 (NZ $66) swimsuit has a studded neckline and bardot sleeves, with a cut out detail over the the stomach.
Under the product care section the website reads "Dry clean" and "This one involves errands".
Despite being listed as swimwear, the product details section tells customers that it cannot be worn for swimming.
Saleem's tweet amassed more than 1,000 likes as other customers questioned the purpose of the swimsuit.
One tweeted: "Have I been using swimsuits for the wrong purpose all these years?!"
"Buy it now and you'll get a chocolate teapot free," another joked.
The one piece is made from bandage material decorated in the studs, which may be the reason it isn't waterproof.
The swimsuit has been produced by PrettyLittleThing, but its description and washing instructions appear to have been written by ASOS.
A spokesperson for PrettyLittleThing said: "The item in question is actually a 'fashion' piece and more of an item to wear around the pool / at pool parties due to the delicateness of the embellishment on it we would not recommended customers to actually swim in it.
"We'll be working to get the product description amended on ASOS."
MailOnline has contacted ASOS and is awaiting comment.