Travis McLeod, the owner of Fremantle's Finest on Adelaide Street, said he watched countless addicts attend drug programs and rehabilitation programs without success before finding their way back to hard drugs.
He said synthetic or "herbal highs" gave them a different avenue to try.
Mr McLeod said he was offering a "safe and healthy alternative to the hard stuff", but the Drug and Alcohol Office says the substances pose a serious health risk and that users could never tell exactly what was in the drug.
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READ NOW"What I sell are not synthetic copies of illegal drugs. All the compounds are found in many plants and foods naturally and are allowed to be made into dietary supplements," Mr McLeod said.
"It is possible to make herbal highs using compounds that are already found in the foods we eat every day."
South Metropolitan Region MLC Phil Edman said he was "outraged" when told the store was openly selling synthetic drugs.
"It is inconceivable to me that a shop like this, that can do so much damage to families and the community, would be allowed to open in the City of Fremantle," he said. "People need to be protected from the harms of illicit drugs and their synthetic copies, not encouraged to continue using them.
"The need is urgent, now more than ever, for the Government to introduce new legislation that will ban all forms of psychoactive substances."
Drug and Alcohol Office executive director Neil Guard said new legislation would soon be introduced to Parliament that would ban the sale, supply, manufacture, advertising and promotion of psychoactive substances.
"A range of synthetic drugs continue to emerge at a rapid rate across Australian jurisdictions," he said.