The cause of the incident is still being established.
Local police said part of the vessel collapsed, causing it to flood and then capsize.
But the Emir of Patigi told journalists that river waves overtook the boat and forced it to crash into a tree that had washed into the river, causing the boat to capsize.
Kwara state Governor AbdulRahman AbdulRazaq gave his “heartfelt condolences” to the victims’ loved ones – and said rescuers were continuing to search for survivors.
He visited the affected villages on Wednesday and promised to provide them with one thousand life jackets to kickstart a safety education initiative.
He also said he had initiated discussions with the National Inland Waterways Authority on how to better enforce safety regulations.
Nigerian President Bola Ahmed Tinubu said he was “deeply saddened by the news of the tragic boat accident that claimed the lives of our people in Kwara State”.
“That the victims were guests at a wedding ceremony made the unfortunate accident more painful,” he added.
River accidents in this part of Nigeria are common.
People who live in the villages bordering the River Niger – which runs through the centre of the country – often use the river as it can be faster than roads, which are often poorly maintained and dangerous, due to the presence of kidnapping gangs.