A five-year-old boy went on a solo bicycle ride around Brooklyn late Saturday night — and his mother was arrested after it took her nearly 20 hours to report him missing, police said Sunday.

Police found the child around 10:30 p.m. near East 55th St. and Linden Blvd. in East Flatbush while he was zipping around on his black and green bike with training wheels.

The kid couldn’t tell police where he lived and didn’t have any contact information for his parents.

Cops released his photo and a picture of his bike Sunday at around 1 p.m. in an attempt to reunite him with his family.

Around 3 p.m., his mom, Nichol Spencer, went to the 67th Precinct to report him missing, police said.

She told cops that she thought the boy was with his dad.

Spencer, 45, was charged with acting in a manner to injure a child. She was waiting arraignment in Brooklyn Criminal Court Sunday night.

The boy was placed in the care of the Administration for Children’s Services and will be assessed before being returned to a family member, police said.