The deal values Investis at about £50m.
Investis, which currently works for more than 70 per cent of the companies in the FTSE 100 and a total of nearly 2,000 clients across Europe and the US, was previously backed by Gresham, another private equity company.
It follows a series of acquisitions for Investis, whose client roster includes Anglo American, GE, BAE Systems, BMW, Capita, Chevron, easyJet, GDF Suez, Novartis, Prudential and Rolls-Royce.
In August the group announced that it had bought Global3Digital, one of its smaller UK rivals, for an undisclosed sum. Over the past two years, Investis has opened offices in the US (New York, San Francisco and Palm Beach in Florida) and acquired Alert IR in the Nordic region.
It also represents Alibaba, the Chinese online retailer that recently underwent what is reported to be the world's largest IPO.
A spokeswoman told PRWeek that the initial plan would be to consolidate Investis’ newly expanded businesses, with global expansion also planned.
Mid-market private equity firm ECI specialises in management buyouts, buy-ins and development capital deals of growth companies valued between £10m and £150m.
Recent ECI investments include online car rental platform CarTrawler (sold to BC and Insight Venture Partners), online auction firm ATG Media and theatre booking company Encore Tickets.
Helen James, CEO of Investis, which is headquartered in London, said: "We are very pleased to be working with ECI Partners. It has a strong track record supporting fast-growing, innovative companies in tech-enabled businesses services.
"Investis is poised for further expansion. We are making significant inroads in the US and have expanded our European business through our recent acquisitions of Global3digital and Alert IR. We are the international leader in this fast-growing sector."
David Grigson, chairman of Investis, said: "Investis has made great strides since Helen took over as CEO two years ago, and is now a bigger, better and more international business. We are delighted that ECI has chosen to invest in the company’s continuing growth."
Richard Chapman, partner at ECI, said: "We are delighted to be partnering with Helen and her team at Investis as the company moves into its next growth phase. As a growth-focused investor we were attracted to Investis by its market leading technology, strong management team and global platform, attributes that have driven its market penetration."
Investis was advised by Clarity, the media and technology corporate finance adviser.