The former Shamrock Rovers and Shelbourne striker will receive treatment and is "optimistic and positive for the future"
The 31-year-old had been experiencing "relatively minor symptoms" before seeking medical advice and will begin treatment within the next few days. Airtricity Premier Division club Drogheda United released a statement announcing the news and said that they were offering O'Neill and his family "every possible support."
The statement read: "The board of Drogheda United is saddened to have to inform colleagues, friends and supporters that forward Gary O'Neill was yesterday diagnosed with testicular cancer.
"He had been experiencing relatively minor symptoms for a short of period time and thought it prudent to seek medical advice. As a result he was sent for tests which has resulted in [Wednesday's] diagnosis. He will undergo surgery in the next few days and is likely to require further treatment in the coming weeks and months.
"He is hopeful of a satisfactory response to the treatment and he remains optimistic and positive for the future. We are offering Gary and his family every possible support as he undergoes treatment and we look forward to his full and speedy recovery.
"Our thoughts and prayers are with Gary and his family at this difficult time and we know that the many friends that he has made in his long career in football will join us in wishing him well."
Dublin-born O'Neill began his professional career at Bohemians and has enjoyed spells with a number of clubs including Waterford United, Shelbourne, Dublin City, St Patrick's Athletic, Sporting Fingal and Shamrock Rovers.