The victim, a boy aged 17, died from stab wounds to his thigh after a confrontation at a park in northwest Sydney about 4.20pm on Monday.
Another 15-year-old boy was charged with murder the same night.
Police arrested a second 15-year-old over the alleged attack after executing a search warrant at a home near the crime scene about 9.45pm on Tuesday.
He has since been charged with murder and will appear before court on Wednesday.
The first 15-year-old, who cannot be identified for legal reasons, appeared in a children's court on Tuesday.
He did not apply for bail and will remain in custody for the next eight weeks.
In a police affidavit police allege the young person set upon the victim unprovoked and stabbed him in the groin area.
The court also heard the 15-year-old was on police body-worn camera footage telling an officer "I stabbed another boy in the leg".
The death has rocked the community, which occurred in a reserve area, close to several high schools and within sight of multiple people who were walking nearby.
Locals were seen in the area on Tuesday morning, with some laying flowers in honour of the deceased.
Police said those involved in Monday's confrontation were not from the nearby school and officers believe it was not a random attack.
The second teen was refused bail to appear before a children's court on Wednesday.
18°C