Petzäll was found around 4pm on Saturday afternoon on a couch in his mother's apartment in Varberg in western Sweden, according to the Aftonbladet newspaper.
Petzäll's death was confirmed by his parents via the Swedish Drug Users Union (Svenska Brukarföreningen), which also issued a statement on its website.
"A friend is gone. William died today," the organization wrote on its website.
The Riksdag also released a statement announcing Petzäll's passing
According to Aftonbladet, police are carrying out a forensic investigation of the apartment but don't suspect foul play.
While investigators plan to carry out a forensic medical exam to determine the exact cause of death, the Drug Users Union suggested Petzäll died of an overdose.
"We understand that it was drugs, we know the terrible situation in which he found himself. We heard his calls for help," the organization wrote.
Petzäll entered the Riksdag following the 2010 general elections, which resulted in the far-right Sweden Democrats gaining representation in the Swedish parliament for the first time.
Last autumn, he had an acrimonious split with the Sweden Democrats, announcing he was leaving the party to deal with his substance abuse problems, but opted to remain in parliament as an independent.
Less than a month later, Petzäll was involuntarily committed to a substance abuse treatment centre to prevent an overdose.
In May, he was convicted of drugs crimes after testing positive for narcotics-classified substances for which he didn't have a prescription, a ruling he labeled as "idiotic".
"It's ridiculous to punish people who are suffering from addiction," he told The Local at the time.
After breaking with the Sweden Democrats, Petzäll wanted to use his position in politics to focus on drug policy in Sweden and the way drug addicts were treated by the health and criminal justice systems.
"It's horrible that drug addicts are stigmatized in the way they are. You can't stereotype immigrants or homosexuals, but all drug users are lumped together into a group that deserves to be punished with fines or prison," he said.
In June, Petzäll's mother co-wrote an opinion article with the chair of the Swedish Drug Users Union in Aftonbladet in which she criticized how her son had been treated.
"The doctors have said I should be ready for the worst, that the next time he uses could also be his last," she wrote.
"But what makes me frustrated and angry is that there is help that would work for my son in the form of methadone treatment. But he was denied treatment because he hadn't fallen far enough."
News of Petzäll's death also prompted his former Sweden Democrat party colleagues to issue a statement about his passing.
"The news brings great sorrow. We remember William Petzäll as a valued comrade and a rarely-seen political talent. Our thoughts are now with William Petzäll's relatives," party spokesperson Martin Kinnunen said in a statement.
Petzäll was 24-years-old.