Around 60 lambs were stolen from the Lofthouse area during the theft, which happened overnight on Thursday.
Police said the thieves would have needed transportation like a cattle trailer to remove the large quantity of sheep.
PC David Mackay, of North Yorkshire Police’s Rural Taskforce, said: “The sheep were marked with red across their shoulders, which may now have faded. They have been described as not yet ready and still quite small, and so there is the possibility that they may be on land somewhere."
The theft comes after ten lambs were stolen from a property on Otley Road, near Beckwithshaw, Harrogate, some time last week.
Police have warned that stolen farm animals are often used in the illegal meat trade.
Inspector Jon Grainge, of North Yorkshire Police’s Rural Taskforce, added: “Over recent years, North Yorkshire Police and surrounding rural forces have taken an increasing number of reports relating to the theft of sheep.
"The Rural Taskforce has been focusing on this problem, and we have identified that sheep have been stolen, often in ones and twos, sometimes in trailer-loads. Some will be destined almost immediately for the illegal meat trade, while others are ‘hidden’ in open view in fields and barns across a wide geographic area.
“The difficulty with the nature of this crime is that it is not necessarily immediately obvious to farmers, who may only be able to identify their shortfall at the time of bringing sheep in to scan or to worm, which often gives a very wide timeframe for us to work with. Lines of enquiry are limited, and this is where we need the public’s help to call in suspicious movements of animals, or animals appearing in fields unexpectedly.”
North Yorkshire Police are also continuing to urge people in rural areas to be vigilant, and report any suspicious activity to the police, as part of its Call It In campaign. Even if it doesn’t need an immediate response, every piece of information helps officers build up a picture and could prove vital in bringing criminals to justice.
“If you have any information regarding the theft or are aware of sheep with this marking suddenly appearing on land please contact North Yorkshire Police on 101 quoting 12170128773 or email me, [email protected]”