Sani Ahmed Yerima, 49, is said to have paid $100,000 as dowry for the 13-year-old girl.
But he has defended his decision to marry a fourth wife even though the prospect of prosecution looms.
"The Prophet Muhammad married at the age of nine, therefore any Muslim who marries a girl of nine years and above, is following the teachings of the prophet," Yerima told Al Jazeera's Yvonne Ndege.
"If there's anybody who'll tell me that what you did contradicts Islam, I 'll submit - and I'll do whatever they ask me to do."
Nigerian human-rights organisations say Yerima has broken the law and have called for his immediate arrest.
"If you have a law that says you cannot marry a 13-year-old. If you have a law that says you cannot marry a-15-year old, then why on God's green earth will a guy who is a senator, who makes laws, violate or break that same law?" one rights-group official said.
"It's because he thinks the law is not applicable to him."
Yerima's problems have been compounded by the fact that the wedding took place in the nation's capital, Abuja, where it is illegal to marry under the age of 18.
But in Nigeria's predominantly Muslim north, weddings of underage people do not often cause controversy.
Indeed, many northern states have refused to recognise the law making marriage for people under 18 illegal.
They say that the ban is against the teachings of Islam.
Source: Al Jazeera