Called the Orb, the mark is used to identify tweed made from cloth woven by hand in the Western Isles with wool yarn from island sheep.
Last year, the trademark was officially recognised as a coat of arms.
Edinburgh-based SnapDragon Monitoring has been commissioned to check ecommerce, social media and auction sites for sales of counterfeit tweed.
Lorna MacAulay, chief executive of the Harris Tweed Authority, said: "It has taken generations to build the Harris Tweed brand into the popular global phenomena we see today.
"It is our job to guard against unauthorised use of the brand and we take that role very seriously."
The Harris Tweed Authority has appointed a firm to monitor for misuses of its legally-protected trademark.