Officers were called over concern for a person in a river, believed to be the River Clyde, at Hazelbank, Lanark, at about 5.55pm on Saturday.
Emergency services attended and a body was recovered from the water.
Formal identification is yet to take place, but it is understood to be a 13-year-old boy. His family have been made aware.
A police spokesperson said a report will be submitted to the Procurator Fiscal.
The fatality is one of several drowning incidents that have been reported over the weekend.
Three people drowned and a seven-year-old boy is fighting for his life after getting into difficulties in Loch Lomond on Saturday.
A major incident was declared shortly after 6:40pm after reports of concerns for people in the water near Pulpit Rock at the Ardlui end of the loch.
The schoolboy, a man, 41, and a 29-year-old woman were pronounced dead by paramedics, and a seven-year-old boy was rushed to the Royal Hospital for Children in Glasgow and is in intensive care.
Police Scotland said formal identification of the victims is yet to take place but relatives of the casualties have been informed.
An 11-year-old boy was pulled from the water at the Alexander Hamilton Memorial Park in Stonehouse on Saturday afternoon.
Emergency services were alerted at about 4:30pm, but Police Scotland said the child died at the river.
The incident came a day after Connor Markward,16, from Glasgow, died in Loch Lomond. He got into difficulty in the water near the pier at Balloch Country Park on Friday, sparking a major search.
People across the UK have been warned to take extra care swimming in open water after dozens drowned during recent hot weather.
The Royal Life Saving Society described this week's deaths as "devastating" and urged people to choose supervised spots to cool off.
A spokesperson at Scottish Fire and Rescue Service said: “We are urging everyone to take care around water. If you find yourself in trouble, stay calm, extend arms and legs and float on your back, shout for help. Make sure everyone is aware of the risks and please stay safe in this warm weather.”