Church law says a pope's resignation is valid only if he takes the decision in full freedom and without pressure from others.
And in one of the few times he has broken his silence since stepping down nearly a year ago, Benedict, 86, said: "There is absolutely no doubt regarding the validity of my resignation from the Petrine ministry."
His comments were made in a letter to the Italian website Vatican Insider.
Benedict, who now has the title Pope Emeritus, said: "The only condition for the validity of my resignation is the complete freedom of my decision. Speculation regarding its validity is simple absurd."
Benedict announced his decision to resign on February 11, 2013, and formally stepped down on February 28, becoming the first pope in 600 years to do so.
Two weeks later, Francis was elected the first non-European pope in 1300 years.
Benedict said at the time he was stepping down because he no longer had the physical and spiritual strength to run the 1.2 billion member Church and his decision had been taken in full freedom.
Earlier this month on the day after the first anniversary of the announcement of the resignation, Italian newspaper Libero ran a long story reviving speculation Benedict may have been forced to resign because of scandals in the Vatican.
In 2012, Benedict's butler was arrested for leaking sensitive documents alleging corruption among Vatican prelates and irregularities in Vatican finances.
Libero also suggested Benedict chose to continue to wear white because he still felt like he was a pope.
Benedict, who lives inside a former convent on the Vatican grounds, was also asked about this and responded: "I continue to wear a white cassock and kept the name Benedict for purely practical reasons. At the moment of my resignation there were no other cloths available. I wear the white cassock in a visibly different way to how the Pope (Francis) wears it."