Electronics retailer JB Hi-Fi has apologised and is investigating accusations a Brisbane store refused entry to a young man with Down syndrome.

A Facebook post written by the man's sister has gone viral, attracting more than 48,000 shares and 18,000 comments on the social media site.

JB Hi-Fi Credit:Glenn Hunt

Victoria Milne claimed her brother James was blocked by a security guard from entering the Mt Ommaney store on Monday.

Ms Milne has been contacted for comment.