The other protagonist of this work is a retired military doctor who once served in the former Fifth Northumberland Fire Regiment. He is Sherlock Holmes's roommate, best friend and assistant, and shares apartment 221B on Baker Street with him.He was shot in the left shoulder during the battle, leaving a psychological barrier that led to a limp, but in the episode "A Study in Pink", he and Sherlock tracked the murderer and overcame the psychological barrier and recovered while traveling through downtown London.

He has a steady, gentle personality, and is friendly to others. He is decisive and strong when assisting in solving crimes. Usually, he helps Sherlock run errands to investigate or catch criminals together.His shooting skills are very accurate. When Sherlock's life was hanging by a thread, he shot the murderer through two windows.He admires and trusts Sherlock very much. Although he complains about Sherlock's behavior, he is still willing to help him. Every time Sherlock gets into trouble due to investigation, Watson always helps him explain.

Sherlock became famous because Watson blogged about many of the cases they handled.In S2E3, he was deeply shocked after witnessing Sherlock falling to his death, and temporarily left Baker Street.Married to Mary Morstan in S3E2.