A man, aged 30, has been arrested on suspicion of murder following the incident at about 1.25pm at the Southport, Chapel Street branch, BBC News reports.
Police vehicles and two air ambulances attended the scene after receiving reports that a woman had been injured. She later died from her injuries in hospital.
A Tui spokesperson told BBC News: "We regret to confirm that a female member of staff at our Southport Tui retail store tragically died in an incident today.
"We send our deepest sympathies to the friends and family of the staff member involved.
"We’re doing everything possible to assist the local police with their investigation and support our customers and staff at this difficult time."
Merseyside Police believe the incident was "domestic related". The woman’s next of kin have been informed.