The incident took place in Lydstep Park just before 7pm on Sunday, May 31.
A 16-year-old and a 17-year-old were taken to University Hospital of Wales for treatment and have since been discharged.
The teenager, from Caerau, was arrested on suspicion of grievous bodily harm with intent, possession of a knife, possession of cannabis with intent to supply, and assault by beating.
Det Insp Phil Marchant of South Wales Police said: “The arrest of this 17-year-old male is a significant development in the investigation and is the result of extensive enquiries since the incident was reported.
“We would like to thank the local community for the support and information we have received.
“The victims and their families have been updated and the investigation is continuing.”
A 61-year-old man was also arrested for affray during the investigation on Wednesday, June 3.
Police are appealing for witnesses or anyone with CCTV footage to come forward.
They can contact South Wales Police quoting reference 2600170643.
Enquiries remain ongoing.
