The agency replaces Buchanan in the role.
Earlier this week Plus500, which allows people to bet on financial markets through so-called contracts for difference (CFDs), reported a 151 per cent rise in income to $162.4m (£101.2m) for the nine months to 30 September. The number of people using the service worldwide grew 35 per cent to 85,101 in that period.
Gal Haber, chief executive of Plus500, which operates in more than 50 countries, said: "Over the past year since our listing in London, the company has continued to go from strength to strength and is one of the fastest growing in the marketplace.
"We are excited to welcome the Luther team on board for the next stage in our development as we continue to consolidate our position in core markets as well as expand into new ones."
Luther Pendragon director Harry Chathli said: "We are delighted to have been appointed by Plus500. The company has exciting growth prospects, and we look forward to working with it, providing media and investor relations services to support its growth."
Luther Pendragon operates across a range of disciplines including public affairs, financial PR and international comms. Its clients include global corporations, private companies, trade associations, charities and public sector organisations.