Powerscourt won the account following a competitive pitch.
It follows the arrival of head of comms Laura Harvey, who was formerly head of comms at fashion retailer Jacques Vert Group. She joined Auto Trader in March to oversee the internal and external communication strategies for the business.
Auto Trader's IPO is among the biggest in the UK so far in 2015, with the business valued at around £2bn when it floated on the London Stock Exchange in March. It now has a market capitalisation of £3.5bn, having risen over 35 per cent since listing.
Harvey said: "Auto Trader is a major brand in the UK and my job is to make sure that our communications to our consumers, retailers and employees are best in class and help reinforce our position as the go to site when looking to buy or sell a new or used car.
"Having listed in March, we can now add investors to that list, and I am confident that the team at Powerscourt will be able to help us communicate as clearly to the financial audience as we do to our other stakeholders."
Instinctif previously worked with Auto Trader in a separate brief overseeing PR during its IPO.
Auto Trader, which was founded almost 40 years ago, ceased its print format in 2013 and has been online only since.