The officer was taken to hospital where his condition was described as "stable", a Merseyside Police spokesman said.
He said the force was investigating an "assault of a member of staff at HMP Altcourse on Wednesday".
The BBC understands the attack took place in a transfer area and the inmate was on remand facing serious charges.
Director for HMP Altcourse Steve Williams said: "We continue to support the officer and his family."
A prison officer has had his neck slashed by an inmate at a prison on Merseyside, the BBC understands.