At around 12.15am today (19/4), the offender knocked on the door of a property in Crowthorne Road.
The victim, a woman in her nineties, answered the door and the offender forced their way into the flat, pushed the victim and took her phone.
The victim was then threatened and the offender stole money from her purse before leaving the property.
The victim was not injured.
The offender is described as a woman in her twenties, wearing red tartan joggers and a dark-coloured waterproof jacket. She also spoke with an Irish accent and had brown hair in a pony tail.
Investigating officer, PC Naomi Padgett, based at Loddon Valley police station, said: “We are appealing for anyone who may have witnessed this incident to please come forward.
“We’d also ask anyone in the local area who has personal CCTV to check any footage in case it may have captured something that could assist the investigation.
“Anyone with information can contact police by calling 101 or making a report online, quoting reference 43220166960.
“Alternatively, if you wish to remain anonymous, you can contact the independent charity Crimestoppers on 0800 555 111 or via their website.”