Lena Wilson will take on the role in a "personal capacity", in addition to her £200,000-a-year post as chief executive of the taxpayer-funded economic development agency.
The appointment was endorsed yesterday by Scottish Finance Secretary John Swinney and Scottish Enterprise chairman Crawford Gillies.
But Willie Rennie, leader of the Scottish Liberal Democrats, last night called for an investigation into Ms Wilson's appointment as a non-executive director of London-based international quality and safety solutions group Intertek.
He said: "For one of Scotland's highest-paid civil servants to even consider taking such a considerable annual payment is really insensitive when businesses are struggling to keep afloat.
"It is highly questionable. Clearly this has had the endorsement of John Swinney. He must have lost his senses. We need an immediate investigation into how this came about.
"Any investigation also needs to look at any possible conflict of interest over Ms Wilson's appointment."
Robert Oxley, of pressure group TaxpayerScotland, said: "Many small businesses struggling in the recession will find it incredible that someone already making so much at a full-time public sector job, which is supposed to mean helping them grow, is taking on new non-executive roles as well and topping up her earnings. There will also have to be serious external scrutiny to ensure this doesn't lead to serious conflicts of interest."
However, the Confederation of British Industry welcomed Ms Wilson's appointment. CBI Scotland assistant director David Lonsdale said: "Lena Wilson is a very able individual and this appointment will strengthen diversity in the boardrooms of our larger companies."
A spokesman for Scottish Enterprise said Ms Wilson had stepped down from her board position at The Prince's Scottish Youth Business Trust, a charity that provides finance and professional support to 18 to 25-year-olds to run their own business, to "allow her more time" to focus on the Intertek role.
A spokesman confirmed this was the first directorship of a publicly quoted company taken up by a serving chief executive of Scottish Enterprise.
He said: "This is the first time one of our chief executives has taken on such a role and been offered such a role while in post."
Mr Gillies said "it is more unusual for leaders in the public sector to take up private sector positions", but described the appointment as "excellent news for Lena, for Scottish Enterprise, and for Scotland". He added: "Her Intertek contract requires around a day a month, which she will deliver in addition to her contracted hours as CEO of Scottish Enterprise.
"While Lena will take on this role in a personal capacity, I firmly believe it will benefit her as CEO of Scottish Enterprise. It will build on the commercial awareness and expertise she already has and boost her credibility even further when dealing with business leaders in Scotland and across the world to support Scotland's economic growth."
A spokeswoman for Intertek admitted that Scottish Enterprise had awarded "on average around £10,000 per annum in grants to projects which Intertek has been a sole or joint recipient in".
Scottish Enterprise said if Intertek sought any financial support from the agency Ms Wilson would "remove herself" from the decision-making to avoid a conflict of interest.
A spokesman said Ms Wilson, who had a pension pot with a cash equivalent transfer value of £967,000 at March 31, 2011, worked "significantly more" than her contracted hours at the agency. He said she worked "extensively" in the evening and at weekends, citing her business travel overseas and attendance at various engagements.
Asked whether the one-day-a-month commitment at Intertek meant people would have to deputise for Ms Wilson more frequently, the spokesman replied: "We will use the same processes as we do for Lena's other existing activity and commitments. When she is on overseas travel, we have people who deputise for her during those times. Likewise when she is on annual leave."
Ms Wilson was unavailable for comment but the development agency said her appointment involved a headhunter, and that she had "seven or eight" interviews for the post.
The Federation of Small Businesses in Scotland said: "Scottish Enterprise has a big job to do and we would hope that this appointment doesn't have any sort of interference in the task ahead, growing the Scottish economy."
A spokeswoman for Mr Swinney said the Intertek appointment would "add to Lena Wilson's considerable experience".