Kevin Beaulieu pleaded guilty last month to aggravated assault for the unprovoked attack, which happened in June 2012 at Kent Institution in Agassiz, east of Vancouver.
Court heard Beaulieu walked up to Charmaine Weiss, asked her to explain something on a piece of paper he was holding and, when she looked down, used a razor blade to slash her across the face.
Weiss was transported to hospital where she underwent a three-hour surgery and has a scar that continues causing her pain.
B.C. Supreme Court Judge Murray Block sentenced Beaulieu, who is already serving a seven-and-a-half year term, to an additional seven years.
Blok says Beaulieu, who has a lengthy criminal record, chose a victim at random to carry out a pre-meditated attack and then bragged about it to fellow inmates.