Officers were called to Harehills Lane at 3.42pm. The injured male was taken to hospital but died a short time later.
A 15-year-old male has been arrested on suspicion of murder and remains in custody.
Detective Superintendent Pat Twiggs of Protective Services (Crime) is leading the investigation and is appealing for any witnesses or anyone with information to come forward.
Detective Superintendent Pat Twiggs said: This tragic incident happened in a busy area at a busy time of day with large numbers of people going about their daily business. I am appealing directly to anyone who witnessed the incident or has information that could help our enquiries to come forward. We are particularly keen to speak to anyone who witnessed anyone running in the area to come forward.
It is also possible that people may have recorded footage on their mobile phones and I would again encourage those people to come forward.
A large police cordon remains in the Harehills Lane area and forensic examinations are likely to continue over the next couple of days. There is likely to be some traffic disruption as a result.
Anyone with information should ring the police on 101 regarding log 1017 of 10th or ring Crimestoppers on 0800 555111.