Sarah Reed, 32, was found dead in her cell at Holloway Prison, north London, on 11 January. She had been on remand for a wounding with intent charge.
Met Police officer James Kiddie was found guilty of common assault in 2014 following an attack on Ms Reed during a 2012 shop-lifting allegation.
The Ministry of Justice said Ms Reed was "found unresponsive in her cell".
It said: "Prison staff attempted CPR, but she was pronounced dead shortly after.
"As with all deaths in custody, the independent Prisons and Probation Ombudsman will conduct an investigation."
PC James Kiddie, 45, was caught on CCTV grabbing Ms Reed by the hair and then punching her as she lay on the floor of Uniqlo, Regent Street in 2012.
He was later dismissed from the force.
He was sentenced to a 150-hour community order for common assault.
Ms Reed's death has prompted a surge of online grief and anger on Twitter urging the authorities to investigate her death further.
Correction 5 February 2016: This article has been amended to clarify the offence for which Ms Reed was on remand.
An investigation has started into the death of a prisoner who had previously been the victim of police brutality.