Doncaster Central Neighbourhood Policing Team has issued the alert with more people discovering the countryside during the coronavirus lockdown.
A spokesman said: “Since the start of Covid we have all become more interested in walking out in the more rural areas of South Yorkshire. In doing this you are more than likely to come across fields with livestock in them such as sheep, cows and horses.
"Most animals will not come near to you as they are not domesticated, however, horses will more than likely approach when they think you have something for them such as food.
"Over the past few months there have been reports on social media of horses being made seriously ill and sometimes dying because members of the public are feeding them.
"Sometimes, horses just like humans, need to have a strict diet to keep them fit and healthy and these needs will be taken care of by the owner.
"Our message is please do not feed the horses when you are out and about, your kindness may be killing them.”
And the warning comes after a national surge in horse deaths in the last year from colic - a painful and life-threatening condition caused eating too much of the wrong food like bread, carrots and apples.
A national campaign is also underway by Facebook group 'Stop Feeding Our Houses' to educate the public on the harm it is causing, and has grown to over 9,000 members in just two months.