According to the BZ newspaper, the victim Sigfried L., 57, was riding his bike through Schiller Park in Berlin's Wedding district on Sunday afternoon and decided to take a rest on a bench.
But when he touched a nearby plastic bag, it exploded and shrapnel hit him in the face and legs.
An ambulance took him to a local hospital, where he underwent surgery for his eye injury. His life is not in danger but he remains hospitalized, the BZ reported.
Police described the explosive device as a homemade pipe bomb filled with metal shards or nails. Sniffer dogs searched the cordoned-off park for more bombs but couldn't find any.
A similar bomb was founded in a bag nearby in May but it was defused by police and never exploded, authorities said.