Both companies offer digital services including websites, mobile phone apps and share price feeds.
Global3Digital designs, builds and supports corporate websites and apps and works with 150 companies across the UK and Europe.
Meanwhile, Investis provides similar services but works with 1,700 clients from offices in six countries.
Helen James, CEO of Investis, said: "We are delighted to acquire Global3Digital at a time when companies are increasingly focused on digital channels as their primary means of communication. The services provided by Global3Digital are directly comparable with our own, as is its commitment to great customer service. We look forward to maintaining that excellent service as we welcome its clients to Investis."
Global3Digital will retain its brand identity for the time being but will eventually be subsumed into Investis, the company said.
Peter Kemp, MD of Global3Digital, will work with Investis during a handover period of a few months before leaving the company.
He said: "We are delighted to be joining forces with Investis. Our clients will continue to benefit from great customer service and from the strong product development and innovation for which Investis is known."