At around 9:30 p.m. on Wednesday April 2, Katie McGregor’s bag was stolen from outside The Carpenters Arms on Whitfield Street in central London.
Inside the bag was a hat belonging to her daughter who died not long after she was born last January.
She said she put her bag down to buy a drink and when she looked again, "it was gone, within literally seconds".
The #findisobelhat campaign has now been launched to try and help jog people's memory so they keep an eye out if in the area.
A designer from the Facebook group has also designed posters and a printer now has donated 500 flyers and 100 posters.
McGregor is planning on heading back to the area soon with the posters and "as many people as possible".
She added the manager of the Carpenters Arms, Danny, has been "absolutely fantastic" by searching through all the local side streets, checking bins and speaking with the other managers in the area.
Anyone who has any information about McGregor’s stolen bag or her daughter's hat is urged to get in contact using the information on the poster above.
The campaign group can be reached on Facebook or by email: email@example.com.