The schoolboy was seen getting into the River Don in Mexborough on Saturday, but he was not seen getting out, police said.
South Yorkshire Police were called to Ferry Boat Lane at about 8pm, and a major search operation was subsequently launched.
Doncaster Response Team Inspector Neil Pryce said: "We know, from witnesses and items left on the riverbank, where the boy entered the water.
"Utilising specialist knowledge and skills, a coordinated multi-agency operation is under way and we are doing everything we can to find this young man.
"We are invaluably assisted by an underwater search team, supported from above by the police helicopter and drones.
"This is a tragic incident and we understand that members of the public want to do all they can to help but we must ask that, for everyone's safety, only those who are involved in our operation enter the River Don."
In addition to police, the search also included personnel from the National Police Air Service, South Yorkshire Fire and Rescue Service and the Yorkshire Ambulance Service.
Read more from Sky News:
Record-breaking heatwave gives way to cooler weather
Murder investigation launched after woman's death
Specialist rescue crews were seen on the streets of Mexborough on Sunday, as they carried out their search.
The boy's family have been informed and is being supported by specialist officers.
South Yorkshire police asked the public not to speculate on the search and show the family the "dignity they deserve".
The village in south Yorkshire, which has a population of about 14,600, is about 12 miles (19km) from Sheffield.
The search comes amid the deaths of several young people who got into difficulty in open water during the recent heatwave.
The deaths include 12-year-old Junior Slater who died after getting into difficulty in the River Ribble at Ribchester in Lancashire and more than 10 others aged in their teens.
The capture of Beaufort castle, near the city of Nabatiyeh, follows days of airstrikes and intense fighting in nearby villages as Israeli troops battled Hezbollah fighters.
Israel Defence Forces (IDF) soldiers raised the Israeli flag on top of the fortress following its capture, with Israeli defence minister Israel Katz declaring that the country would hold the strategic point.
The seizure came as the IDF issued an evacuation warning for residents south of the Zahrani River, in a move that expands the territory it has sought to clear in southern Lebanon.
It marks a major development in the latest Israel-Hezbollah war, which began on 2 March when Hezbollah fired on northern Israel two days after the US and Israel attacked Iran.
Israel's Prime Minister Benjamin Netanyahu praised the "heroic fighters" behind the capture of the castle on Sunday, declaring that Israel would expand its control in Lebanon.
"My instruction is to deepen and expand our grip on the places that were under Hezbollah's control," he said.
The Israeli military's Arabic-language spokesperson, Avichay Adraee, posted pictures on X showing Israeli troops walking outside the castle.
The Beaufort fortress, perched high above Lebanon's rolling green hills and overlooking the Litani River, has been a strategic military asset for armies over the last thousand years.
Built as a Crusader castle around the 12th century, it has been used by Saladin's Jerusalem army, the Ottomans, the Palestinian Liberation Organisation and the Israeli military.
Read more from Sky News:
Rain threatens search for two missing in Laos cave
WHO chief heads to Ebola epicentre
Israeli troops previously captured the castle in 1982 and held it until they withdrew from Lebanon in 2000.
The Crusaders named it Beaufort, which is Old French for "beautiful fortress".
Israel's push into southern Lebanon came despite a nominal ceasefire being in place since 17 April.
The IDF are now about three miles (5km) from the city of Nabatiyeh, a major centre in southern Lebanon, and has called on all its residents to leave.
Residents of the coastal city of Tyre, the country's fourth largest city, and its surroundings, have also been told to leave.
The next round of talks between Israel and Lebanon are due to be held on 2 and 3 June.
The incursion has so far killed ‌more than 3,300 people, the Lebanese government said.
At least 25 Israeli soldiers and a defence contractor in or near southern Lebanon, including one on Saturday, according to Netanyahu's office. There were also two civilians killed in northern Israel.
There was no immediate comment from Hezbollah or the Lebanese government on Israel's seizure.
A 39-year-old woman was arrested in Hounslow, London, on Friday on suspicion of murder after the address of the van's keeper was identified.
Following the arrest, and as a result of other enquiries, the Metropolitan Police took over the case on Saturday 30 May and established a crime scene at a house in Sutton Dene, Hounslow.
Leicestershire Police were originally called to a car park in Garden Street, Leicester, just after 5.30pm on Wednesday, 27 May, by a member of the public reporting concern about the condition of a white Ford Transit.
When officers forced entry to the van, the body of a man was found inside. Police are now trying to locate the man's next of kin.
An investigation to establish the circumstances that led to the man's death is still ongoing.
Read more from Sky News:
Meteor travelling at 75,000mph shakes US buildings
WHO chief heads to Ebola epicentre
Detective Chief Inspector Alex Gammampila, leading the investigation, said: "I'd like to thank Leicestershire Police for their initial enquiries, which led to a swift arrest.
"As we piece together the circumstances surrounding this tragic incident, we urge the public to come forward with any information, particularly if you recognise the van, or think you may have captured it on a doorbell or dashcam footage travelling to Leicester in the lead up to Wednesday."
Marc Bolland has been chosen by Work and Pensions Secretary Pat McFadden to deliver the "youth guarantee" for every young person aged 18 to 21 to have access to employment, training or education.
Speaking on Sky's Sunday Morning with Trevor Phillips, Mr McFadden said that "the experience that he'll bring to this will be really valuable".
Earlier this week, a review by former Labour health secretary Alan Milburn found that one in eight 16 to 24-year-olds are not employed and risk becoming a "lost generation" without urgent intervention.
Mr Milburn also warned of a moral and economic "crisis" posed by the rising number of NEETs.
Meanwhile, figures from the Office for National Statistics suggest youth unemployment reached 16.2% in the three months to March - the highest level in 11 years.
Mr McFadden said Mr Bolland's appointment "sends a clear signal that we are serious about tackling that challenge".
"His track record in business and through Movement to Work make him uniquely placed to bring employers together and open up real opportunities for young people who need them most," he added.
As well as running Marks & Spencer, Mr Bolland was chief executive of Morrisons and chief operating officer at Heineken.
In 2012, he founded the charity Movement to Work in response to the previous year's riots, helping more than 200,000 disadvantaged young people into work.
Read more from Sky News:
Number of heatwave open water deaths rises to 15
Police mount investigation after rare miniature horses stolen
Speaking about his new appointment, Mr Bolland said: "I believe the government is serious about tackling this generational crisis of youth unemployment, and I know that working hand in hand with business to support young people gives them the best possible chance of success."
After the release of Mr Milburn's report, the government announced some of Britain's biggest businesses would back 300,000 work experience and training placements - from construction to hospitality - for young people over the next three years.
Crews have already rescued five other people who became trapped underground 11 days ago.
Finnish diver Mikko Paasi, one of the first international rescuers to arrive at the site in the central province of Xaisomboun, said the rain had filled the cave up to the second chamber.
The rising water level means no diver will be able to enter the cave until pumps can lower the water level.
The seven villagers became trapped after they entered the cave to look for valuable minerals and a flash flood blocked their way out.
Another villager escaped and alerted authorities.
Rescue teams from Laos and neighbouring Thailand have been working together as part of the rescue operation.
The crew was also joined by divers from Finland, Malaysia, Japan, Indonesia, France and Australia.
The cave is in a rugged area about 75 miles (120km) north of the capital, Vientiane, and the cave's entrance requires a steep 2.5-mile (4km) hike.
Several of the rescuers previously took part in the operation to save 12 schoolboys and their soccer coach from a flooded cave in Thailand in 2018.
Malaysian diver Lee Kian Lie said the men who were retrieved from the cave are being treated at a local hospital and are being interviewed about the challenges crews face in retrieving the final two men.
"We interviewed them about how the deeper part of the cave looks like. We will continue to search based on the information we have, and perhaps we will be able to get to the other two," he said.
Read more from Sky News:
Kanye West banned from performing in Italian city
Australian mother appeals mushroom murder conviction
Rescuers navigated more than 656ft (200m) into the cave and discovered five chambers within the system. Those rescued so far were found in the fifth chamber.
Mr Paasi said survivors had told crews there was a narrow crack in the fifth chamber that could lead deeper into the cave.
"This was the only place that we haven't checked in the mine, where the two lost miners could still be," he said during a video interview.
"Now there's a theory that, through that small crack, it still continues, and there's a sixth chamber, which gives us hope now that, if we could penetrate that small restriction, we might be able to reach the sixth chamber and then see what is there."
Of the five rescued, one man was guided through the narrow flooded passage by an expert diver on Friday and four left the cave the next day once the water level had receded enough to exit the cave.
It's believed the men entered the cave on 20 May.