The woman was taken to hospital after the incident involving a male inmate at Maghaberry Prison, near Lisburn, on Monday.
Adrian Smith from the Prison Officers Association (POA) told BBC News NI the woman was one of the newer prison officers.
He said he believed she was attacked with a weapon "made from a razor".
"She has an approximately three inch deep cut below her left ear," he said.
"With the ever increasing budget cuts, I believe this will happen more often," he added.
A Prison Service spokesperson said: "The Prison Service utterly condemns this attack and has referred the incident to the PSNI".

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Qatar's Femi Ogunode was also given the same time, but Gatlin got the nod with France's Jimmy Vicaut third in 9.99.
Briton Chijindu Ujah finished eighth in Brussels, in a time of 10.19.
American Gatlin, 33, has run personal bests for the 100m and 200m this season but finished runner-up to Usain Bolt in both events in the World Championships.
The Jamaican opted to end his season rather than run in Belgium.
Gatlin was also due to run in the 200m, but pulled out. That event was won by Ogunode in an impressive 19.97.
In one of the races of the night, Dutch 2015 world champion Dafne Schippers beat Olympic champion Allyson Felix to win the 200m in 22.12.
Briton Jodie Williams was seventh in 23.34.
Williams's compatriot, Paralympic champion Jonnie Peacock, eased to victory in the T44 100m with a time of 10.99. Germany's Felix Streng was second in 11.08, while world record holder Richard Browne was disqualified for a false start.
There was more British interest in the 100m hurdles, with Tiffany Porter taking fourth spot in 12.81 behind winner Dawn Harper-Nelson of the USA who clocked 12.63.
A prison officer has suffered a serious neck wound after she was attacked at a jail in County Antrim.
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Justin Gatlin was crowned overall Diamond League champion in the 100m after winning the final race of this year's series in 9.98 seconds.