The incident happened around 7:30 p.m. on the Crenshaw station platform, located in the median of the 105 Freeway near 120th Street.
It's unclear what led up to the stabbing. Authorities initially said the victim was wounded in a shooting.
A person of interest was detained, authorities said, but further details about his involvement were not immediately available. The person of interest was taken to a hospital with unknown injuries.
The victim was rushed to a hospital but later died.
Deputies were also seen investigating an area near an onramp of the freeway.
Metro train service between the Hawthorne/Lennox and Vermont/Athens stations was shut down and bus shuttles were provided instead as law enforcement's investigation continued in the area.