The theft happened in Greenhead Park, on the edge of Huddersfield town centre, at around 4pm on Monday.
Police said the victim suffered a medical episode while sitting on a bench in the park near the small lake.
The thief stole her bag - which contained camera equipment and a mobile phone - and stole more than £1,000 worth of property from the woman.
Enquiries are ongoing, a spokesperson for West Yorkshire Police said.
Anyone with information is asked to contact police via 101, quoting crime reference number 13170520325.