The 22-year-old woman, visiting the market at Kungstorget, asked the person selling the mushroom what it was, but he did not know. So she took a picture and contacted the police.
When the newspaper Göteborgs-Posten showed the picture to mushroom expert Magnus Neuendorf, at the Botanical Gardens in Gothenburg, he was in no doubt: this was amanita regalis, a brown version of its more notorious, and similarly poisonous, red cousin, the fly amanita.
"If you eat it you will get very sick and start hallucinating," Neuendorf told the newspaper Göteborg-Posten.
Symptoms will appear pretty quickly, often within half an hour or two. They are not life threatening, but the person who has eaten it may need hospital treatment.
"During my 30 years as a mushroom expert, I have never heard of anything like it," Neuendorf told the paper.
Author and mushroom expert Pelle Holmberg tells Radio Sweden that there needs to be more scrutiny of mushrooms sold at market.
"It's sad that the National Food Agency have not done anything about this. They need to check the stalls and make sure the mushrooms are safe."
Gothenburg municipal council sent two inspectors to the market concerned on Tuesday.
Six people in Varberg were poisoned by the white 'flugsvamp' out in the forests at the weekend.
The Swedish Poisons Information Centre receives on average 2,000 calls a year from mushroom pickers worried that they may have eaten the wrong kind.