Onovwigun, 18, impressed after playing for the Spireites on trial in pre-season. He has agreed a one-year contract, with the club having an option for a further 12 months.
Manager Paul Cook said: "We believe that Michael has a lot of potential and we view him as one for the future. Our chief scout, Paul Mitchell, brought him to the club after being alerted about his availability by Mike Appiason, who is one of our contacts in the London area.
"He was initially on trial with the development centre squad and we quickly came to the conclusion that it was worth offering him a professional contract. It is a big year for Michael, who now has a chance to fulfil the potential he showed at Brentford."