The dad-of-two, who won an Olympic gold medal for long jump at London 2012 was eventually checked by a doctor and told he had cysts and it was "nothing to seriously worry about".
In a lengthy Instagram post, the 33-year-old said he initially ignored the lump - distracting himself with training and using his "physical strength to overpower the unknown".
He continued: "Obviously that didn’t work and my mental health took a bit of a beating when fear set in".
The retired athlete kept the lump a secret from his wife and loved ones.
He eventually confided in a friend - who had suffered from severe testicular cancer - that friend encouraged him to see a doctor.
Rutherford is now urging men to check themselves for lumps, saying: "Even now, during a pandemic, when I think it’s safe to say we’re fearful of wasting doctors and nurses time.
"If you’re a bloke, grab them and make sure nothings wrong. And if your partner won’t check their own balls, maybe offer to do it for them."
Testicular cancer is one of the less common cancers and typically affects men aged between 15 and 19-years-old.
The NHS website says common symptoms are a painless swelling or lump in one testicle, or a change in the shape or texture of the testicles.
It adds: "It's important to be aware of what feels normal for you. Get to know your body and see a GP if you notice any changes".
For more information on testicular cancer visit the NHS site.