Police officer Aigars Berzins said the incident took place Oct. 3 in an apartment building.
The dog had unexpectedly attacked the man's friend and as its owner tried to pull the dog off, it attacked him also.
The dog will be killed because of the attack.
The man who was attacked will undergo treatment for rabies, Riga First Hospital Director Janis Ozols said.
Latvia's Dog Handler Federation head Vija Klucniece was not informed about the incident.
The federation currently is focusing increased attention on dogs of dangerous breeds thanks to new regulations banning dangerous breeds, including the Stratfordshire terrier.
Klucniece said there were Stafordshire terriers in Riga that were raised to protect people and that their behavior and aggressiveness were unpredictable.
Last summer an American Stafordshire terrier in Riga attacked an eight-year-old boy in an apartment building elevator, seriously injuring him. Following the attack, the government decided to ban keeping, breeding and importing such dogs in Latvia.