The Welshman had been having treatment for Testicular Cancer which had spread to his brain and lungs.
Hartson had surgery to ease this and has made a good enough recovery to be allowed to leave the hospital in Swansea where he was being treated.
A Abertawe Bro Morgannwg University NHS Trust statement reads : "John Hartson has completed the first phase of his chemotherapy treatment and has been discharged from hospital.
"He will now continue his chemotherapy treatment as an outpatient of the hospital."
Hartson thanked everyone for their support and told the BBC: "I'd like to thank all the wonderful staff for the excellent care they have been giving me.
"I'm really grateful to all the people who have sent messages, cards and flowers - they have been such a big lift to me and my family.
People have taken time out to write a message and buy a card and that means a lot.
"I'm not going to be able to thank everyone personally for their message but I appreciate every one, both from inside and outside football.
"And the biggest thanks to my fantastic family and friends."