The ambulance service informed police at about 2.55am on Sunday, May 24, that a woman in her 30s had been found dead at a premises in Brook Cottage Close in Peasley Cross St Helens.
A 63-year-old man was arrested on suspicion of murder and has been taken to a police station for questioning by detectives.
In a statement, police said work is ongoing to “establish the circumstances and to formally identify the woman”.
The incident is initially being treated as “unexpected” while these enquiries are carried out.
The full police statement read: “We can confirm that an investigation is underway following the unexpected death of a woman in St Helens in the early hours of this morning, Sunday May 24.
“At around 2.55am, NWAS informed police of a woman in her 30s being sadly pronounced deceased at a premises in Brook Cottage Close.
“A 63-year-old man at the scene was arrested on suspicion of murder and has been taken to a police station for questioning by detectives.
“Work is ongoing to establish the circumstances and to formally identify the woman. The incident is initially being treated as unexpected while these enquiries are carried out.
“Anyone with information is asked to contact us @MerPolCC or call 101 with reference 26000410495. You can also report information via our website: https://www.merseyside.police.uk/ro/report/ocr/af/how-to-report-a-crime/.”
Alternatively, contact the independent charity Crimestoppers anonymously on 0800 555 111, or via their website here: https://crimestoppers-uk.org/give-information/forms/give-information-anonymously