Giffen, which is based in St Albans, specialises in mechanical, electrical and power services in the rail sector, and works for both Network Rail and London Underground plus several train operating companies.
The firm has four frameworks with Network Rail and six frameworks with London Underground.
Giffen is a direct delivery organisation, employing 123 staff and operatives.
Following the acquisition, Giffen, whose entire management team is remaining with the company, will report into Amco Rail, Renew's rail infrastructure business.
For the year ended 30 September 2016, Renew said that Giffen is expected to record revenue of approximately £22m and an adjusted profit before tax of £700,000. It has a cash balance of less than £75,000.
Renew is paying £5m for the acquisition, and a further £2m to redeem loans from Giffen's current private equity owners. It said this will be paid from Renew's existing cash resources.
“By acquiring the complementary skills of Giffen, Amco Rail will be able to offer an expanded range of services across the rail network as well as creating opportunities for the group to provide services to London Underground,” a statement from Renew said.