The incident happened just before 11 p.m. on March 26 on the southbound Q train at Newkirk Avenue in Brooklyn.
Authorities say the suspect approached a 36-year-old man, punched him multiple times and stole his iPhone.
The suspect is described as being in his 20s and wearing a black bubble vest, light colored sweatshirt and red sweatpants.
Anyone with information in regard to this missing is asked to call Crime Stoppers at 1-800-577-TIPS (5477).
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