The 18-year-old allegedly used a kettle in his cell to heat water up and then poured boiling water over the officer on Thursday, the PA news agency understands.
The prison officer was taken to hospital as a precautionary measure, did not require further treatment and was discharged later that evening, it is understood.
They are expected to return to work next week.
Rudakubana was jailed for a minimum of 52 years in January for the murders of three girls and attempted murders of eight other children, class instructor Leanne Lucas and businessman John Hayes.
The 18-year-old murdered Alice da Silva Aguiar, nine, Bebe King, six, and Elsie Dot Stancombe, seven, and injured 10 others during the attack at the Taylor Swift-themed workshop on July 29 last year.
A Prison Service spokesperson said: “Police are investigating an attack on a prison officer at HMP Belmarsh yesterday.
“Violence in prison will not be tolerated and we will always push for the strongest possible punishment for attacks on our hardworking staff.”
The Home Secretary said in April that an inquiry into the Southport murders had formally begun its work looking at how to learn lessons from the attack.
Shadow justice secretary Robert Jenrick said the alleged attack by Rudakubana was the “third astonishing security failure at a top security prison”.
In a post on X, Mr Jenrick said: “Warning after warning has been ignored.
“I personally raised security at HMP Belmarsh just last week.
“This is a full blown crisis. No more lengthy ‘reviews’ – the Justice Secretary needs to act NOW.”
The number of assaults on staff in adult prisons in England and Wales per year has reached its highest level in a decade, according to data from the Ministry of Justice.
Some 10,605 assaults on staff in male and female jails were recorded in 2024, up from 9,204 in 2023 and nearly three times the 3,640 in 2014.
Four guards were attacked with hot oil and homemade weapons by Manchester Arena bomb plotter Hashem Abedi at HMP Frankland in County Durham in April, with the inmate transferred to Belmarsh after the incident.
In response to the rise in attacks, Justice Secretary Shabana Mahmood previously announced that the use of tasers will be trialled in prisons and confirmed the Prison Service will conduct a “snap review” of the use of protective body armour for prison officers.
