Dave Cormack, who was born in Aberdeen and is a lifelong fan of the club, has bought shares and said he will offer "financial support" in addition to working on plans for a new stadium and training facilities.
The businessman has previous experience of working at Pittodrie, having served as interim chief executive from 2000 to 2001 but said he was now able to devote more time to the club after selling his US business.
Mr Cormack was president and chief executive of software company Brightree, which sold for $800m (£629m) earlier this year.
On Tuesday, the Pittodrie club announced the investor was joining their board.
Mr Cormack said: "While I have continued to support and provide advice to the club for many years, I'm honoured to be back on the board."
He continued: "The club's performance in recent seasons, both on and off the pitch, is a testament to how hard everyone at Pittodrie has worked.
"Having a team that is performing well and a club that is debt-free is an enviable position to be in.
"But we must kick-on from here and fully exploit opportunities to increase revenues and our fanbase."
He added: "I'm looking forward to working with the existing, strong leadership team at the club, with whom I've already got a great relationship.
"Between us, and with the support of our fans, I'm sure we can get the club into a position where increased revenues, an expanded fanbase, and a state-of the-art community stadium and training campus will position AFC well to exploit changes that evolve in European club competition."