The hand was found near 5th Street and Fairmount Boulevard in the city's downtown area, according to NBC News, citing local law enforcement.
Later that evening, police learned that a local hospital had admitted a man in his 60s who arrived with a missing hand. The patient reportedly told hospital staff that the appendage "was cut off by someone swinging a sword."
The man is expected to survive his injury, according to police.
Law enforcement in Riverside is investigating the incident, but believe the incident was an altercation between two homeless men.
“This assault may have stemmed from a recent ongoing dispute they have had with each other,” police told NBC News.
No arrests have been made so far.
Sword attacks are uncommon, but they still occur.
Three weeks ago, a court in Connecticut handed down a second 55-year prison sentence to a man who attacked two people with a katana, killing one and severely injuring another.
Peter Manfredonia, 26, attacked and killed Theodore DeMers, 62, with a sword in 2020, according to NPR.
He then attacked John Franco, 80, who came to assist Mr DeMers. The attack left the man with injuries on his hands, neck, and head.
Mr Manfredonia then broke into the home of a third man and held him hostage for 24 hours before driving across the state, where he shot and killed a former classmate and kidnapped his girlfriend. He later let her go at a rest stop in New Jersey.
Police searched for Mr Manfredonia for six days before they eventually arrested him at a truck stop in Hagerstown, Maryland.