Aslin C. Nabarro entered the plea Wednesday.
Other charges related to the assault were dropped.
Nabarro was arrested after a May 17 assault in a parking lot of St. Mary’s Cathedral.
After an argument with a woman identified as Mirna Medina-Colocho, Nabarro stabbed her 20 times in the upper torso and head.
He then cut his own throat and fled but collapsed less than a block away.
Medina-Colocho and Nabarro both recovered.
Nabarro will be sentenced Sept. 26.