There is some debate over who the richest governor in Nigeria is, and this question is made increasingly more difficult by the issue of corruption in Nigeria.
Rochas Okorocha is most people's best bet though.
There isn't much information about how Okorocha came into money, but he is well known for making large philanthropic donations. One source ranks him as 44th on a list of Nigeria's wealthiest billionaires.
It's speculated that many of Nigeria's governors have assets which remain off the books, often due to their questionable origins, making them richer than they officially declare.
However, Okorocha is nowhere near being the richest politician in Nigeria....
He may not be a governor, but former president of Nigeria, Olusegun Obasanjo, is a billionaire with net worth of $1.3 billion. For this reason he is often considered the richest politician in Nigeria.