The discovery came nearly two weeks after Robert was last seen by his family.
A police spokesman said officers were called to woods near Kilbrennan Drive in North Motherwell around 3.15pm yesterday. The area is close to Strathclyde Park.
He added: “The body has not been formally identified, but it is believed to be that of Robert Thomson. Relatives are aware.
“A post mortem will take place in due course to establish the exact cause of death, which is not being treated as suspicious at this time. A report will be sent to the procurator fiscal.”
Robert went missing on Friday, June 3, just five days before his 18th birthday. He was last seen by his dad, Alan, who left him at a bus stop in Motherwell Road, Bellshill.
His mum, Hazel Doonan, said the teenager was in good spirits when he left home and had planned to see a friend in Motherwell.
She sat up until 2am on his birthday, hoping in vain that he would return. Hopes were raised earlier this week when police investigated two reported sightings of Robert in Ayr.