Fontana police responded to a call of a double stabbing shortly after noon in the 13700 block of Sunrise Street.
Police found two victims, a 12-year-old boy with superficial wounds and a 21-year-old woman who was seriously wounded. Both victims were taken to a nearby hospital and are expected to survive.
The suspect, described as a 15-year-old "with diminished mental capacity," was arrested, according to Fontana police. Witnesses said he took off on a bicycle before he was found and taken into custody.
Everyone involved reportedly knew each other, police said. The motive is not yet known.
An investigation into the incident is ongoing.