Speaking at an anti-corruption event in London, Buhari said he was more interested in the return of stolen assets held in British banks.
Mr Cameron made the unguarded comments in a conversation with the Queen.
Nigeria was ranked 136th in Transparency International’s 2015 Corruption Perceptions Index.
Asked at the event if Nigeria was “fantastically corrupt”, Mr Buhari: “Yes”.
In his speech, the Nigerian leader described corruption as a “hydra-headed monster” which threatened the security of countries and “does not differentiate between developed and developing countries”.
The UK government will host world and business leaders at the summit on Thursday in London, aiming to “galvanise a global response to tackle corruption”.
Speaking ahead of the summit, Mr Cameron said: “For too long there has been a taboo about tackling this issue head-on.
“The summit will change that. Together we will push the fight against corruption to the top of the international agenda where it belongs.”