The man, who works as a press assistant in the party and is set to stand in the autumn election, denies the claims.
“We have had discussions with him and he's very clear that he didn't do it. He feels very upset and says he's not responsible for the comments,” a party spokesperson told TT.
The same spokesperson said the Sweden Democrats planned to find out what had happened and that it was carrying out its own internal investigation.
Sweden's Expressen newspaper looked at online comments it claims the man made between 2012 and 2016 on far-right Swedish sites including Avpixlat and Fria Tider as well as the American site Breitbart. The newspaper described the hundreds of messages as “systematically spreading hatred”.
According to the Expressen investigation, the comments were made by an account linked to the politician's private email address, which he also used in contact with Swedish authorities.
The comments included declarations that all traces of Islam in Sweden should be “destroyed” and that Muslims should be deported, as well as describing Muslims as “pure evil” and Arabs as “the scum of the earth”. The person behind the count also posted a positive comment on a report of a mosque fire in London.
“Now let's hope it spreads to Sweden like a wildfire,” the comment on a Fria tider article said.
In an interview with Expressen, the politician said he didn't recognize any of the hundreds of comments made by the account.
The investigation comes just days after another politician in the party was reported for writing on Facebook that Jews and Sami people were “not Swedish”.