The Edinburgh Central MSP, who stepped down as leader in August and is expected to quit the Scottish Parliament at the next election in 2021, has been appointed a senior adviser at Tulchan Communications, which has offices in London and Singapore.
But industry body the Public Relations and Communications Association said there was a clear potential conflict of interest and argued it was wrong for lobbying agencies to employ legislators.
PRCA director general Francis Ingham said: "The possible conflict of interest in doing so is clear, and damages the reputation of both our industry, and of the political process.
"PRCA members are prohibited from employing parliamentarians - and with good reason. Unlike the majority of its competitors, Tulchan is not a PRCA member. But in the public interest, we would nonetheless urge them to reconsider this appointment."
Tulchan's managing partner is Tory fundraiser and former Conservative Party chairman Andrew Feldman, now Lord Feldman of Elstree.
He said: "We are delighted that Ruth has chosen to join Tulchan as a senior adviser and look forward to having her on board.
"I have no doubt that our clients will benefit immensely from her insight and unique perspective on the rapidly developing evolving relationship between business and politics and the need to navigate the shifting demands of a broader range of stakeholders."
In an interview with the Evening Standard, Ms Davidson was quoted saying she would not be lobbying or talking to ministers as part of her new role.
She said: "I am very excited to be joining Tulchan at such a critical time for UK PLC. I believe the evolution of Tulchan's mission statement towards creating stakeholder value is prescient. Political and economic uncertainty persists and the scope for shocks is high.
"As a result, businesses must continue to renew their licence to operate with their stakeholders and reflect on what is expected of them or risk being targeted by anti-business vested interests and losing public faith."
Ms Davidson's resignation as leader came soon after Boris Johnson became Prime Minister. She cited personal reasons and wanting to spend more time with her new baby Finn, as well as being "hopelessly conflicted" over Brexit.