The 72-year-old was approached by the two women outside St Catherine’s Church in Beach Road, Littlehampton on Sunday (July 18), police said.
They asked him to sign their petition and touched his arm before he also realised his watch, believed to be worth around £5,000, was missing.
A 70-year-old was also targeted at a car boot sale in The Cattle Market in Chichester earlier that day.
Detective Sergeant Vicky Guy said: “We understand that reports such as these may cause concern among local communities and we are investigating these incidents as a priority.
“Please be vigilant if approached by strangers who attempt to distract you and report any suspicious behaviour to us so we can take action.”
The first suspect is described as a white woman with a tan, around 22 years old, 5ft 8in, and wearing a light blue coat.
The second is a white woman with a tan, around a similar age, and 5ft 6in.
The Littlehampton victim gave similar descriptions of both women, adding the taller one was wearing a mask, flowery top and long skirt. The shorter woman was wearing a pale dress.
He also reported both women mentioned being from Eastern Europe.
Anyone who may have seen two women with clipboards, has dash-cam footage from either incident or information that could help with the investigation, is asked to report it to police online or by calling 101, quoting Operation Overdrive.
And police are urging members of the public to be vigilant as detectives continue to investigate reports of similar incidents in the West Sussex area.