Mr Hyman quit as the firm attempted to rebuild its relationship with the Government following controversies over its handling of key contracts.
He said: "I have always put the interests of Serco first. At this time, nothing is more important to me than rebuilding the relationship with our UK Government customer.
"In recent weeks it has become clear to me that the best way for the company to move forward is for me to step back. I have been fortunate enough to have had the privilege of working at a great company with extremely talented people. I wish everyone at Serco the very best for the future."
The firm faces investigation after the Government was overcharged millions of pounds for electronically tagging criminals, and there are also allegations of potentially fraudulent behaviour in the management of its £285 million prison escorting contract.
A Government spokesman said yesterday's announcement was a "positive move".
He said: "The Government will take full account of all the changes Serco have made today. This is a positive move by Serco and a step forward."