Stock market listed Impellam, led by chief executive Julia Robertson, will buy the business using a mixture of cash and shares.
Lorien, which specialises in tech recruitment, has offices in London and recruits workers to clients such as Glaxosmithkline and the Royal Bank of Scotland.
The takeover will transform Impellam into the second largest staffing firm in the UK judged on revenue up from its position as third largest. Adecco is the number one.
“It has long been our ambition to develop a substantial presence in IT staffing,” Robertson said.