The incident happened in the village of Eggborough, near Selby, at around 12.30pm on Tuesday September 7.
The two-year-old boy's mother told police that an adult, believed to be a woman, had picked him up from the front garden of a house and walked off with him.
She challenged the woman, who dropped the child and ran off along Selby Road. The boy was unhurt.
A large-scale search operation took place and the National Police Air Service helicopter was called in, but the suspect was not located.
The woman is described as white, of a skinny build, with long dark hair tied back in a bun. They were wearing a blue baseball cap, white shirt with blue sleeves, jogging bottoms and white trainers.
Extensive enquiries are ongoing to identify and locate them.
A spokesperson for North Yorkshire Police said: “This is clearly a concerning incident. Although incidents like this are rare, please be vigilant, and contact us if you have any information that could assist our investigation.”
Anyone with information that could assist the investigation is asked to contact North Yorkshire Police on 101, quoting reference 12210197086.