Timothy Bayliss is a fictional character on Homicide: Life on the Street, played by Kyle Secor and one of the few main characters to last the entire run of the show. He was loosely based on real-life Baltimore homicide detective Thomas Pellegrini, featured in David Simon's book Homicide: A Year on the Killing Streets, though Pellegrini was reportedly not at all a fan of his fictional alter ego. The character also appeared in the Law & Order episode "Charm City."

Born on May 31, 1960, in Baltimore, Maryland, Bayliss had a difficult and often contentious relationship with his family. Growing up he was very close with his cousins Jim and Kurt. In Season 3, after Jim shot and killed a Turkish exchange student, Bayliss tried to shoehorn himself into his partner Frank Pembleton's investigation. Pembleton learned that Kurt was killed during the Persian Gulf War and that Kurt and Jim's father was racist. The case went to a grand jury, which voted not to indict Jim. In Season 4, Bayliss briefly mentioned having a six-year-old niece. His undergraduate minor was in drama.

In Season 5, Bayliss revealed to Pembleton that he had been molested as a child by one of his uncles. When he told his father what had happened, the elder Bayliss accused him of lying and the relationship remained hostile until his father's death. Bayliss told Det. Meldrick Lewis that he was once arrested for protesting U.S. policy in El Salvador when he was a teenager, a story idea that Secor reportedly disdained as out of character for Bayliss. It was quickly discarded in favor of the character developments for Seasons 5–7, including his abuse history and religious journey.

Unlike several characters in the series, religion was not important in Bayliss's family background. When asked, he stated he had been raised "mutt." In this case that meant his family attended several different denominations, most of which could be termed "Mainline Protestant," but they had no attachment to any of them. Bayliss states that he was baptized into the Presbyterian Church and confirmed in the Episcopal Church. He briefly joined Unitarian Universalism for a girlfriend, but seems to have not been particularly sincere about it. He converted to Buddhism in the final season but eventually lost faith.

Tim Bayliss had originally worked for the mayor's security, but his ambition had been to work at Homicide. His first case as primary detective was the rape-murder of Adena Watson, and he was never able to close it. This case haunted him throughout the series, but particularly in the first four seasons. At times, it led to friction between himself and his partner, Frank Pembleton (Andre Braugher), who scolded him for putting too much of himself into his cases. In the Season 4 episode "Stakeout," he learns that Risley Tucker (Moses Gunn), an arabber who was the prime suspect, has died of natural causes. Pembleton and Bayliss had put Risley through a long interrogation in hopes of getting him to admit his guilt, but without success.

The Season 4 episode, "Requiem for Adena", centers on the murder of a young black girl that shows similarities to the Adena Watson case. Bayliss becomes obsessed with the idea that the two cases are connected, to the point that his actions begin to jeopardize Pembleton's efforts to get a confession. He learns that many of the people connected with Adena and/or Tucker have moved on from her death much better than he has, and states that he has begun to hate Adena because he cannot do so himself. At the end of the episode, he takes a framed photo of her from his desk (where it had stood ever since that investigation wound down), packs it into an envelope with a carnation from his lapel, and drops the envelope into a trash can.

In the Season 6 episode "Finnegan's Wake", Bayliss wrestles anew with the Watson case when he learns about the longest-running unsolved homicide on the BPD's books, the rape and murder in 1932 of a little girl named Clara Slone. Pembleton tells Tim that the senior detectives all decided to not tell him about the Slone case because it strongly echoed Adena Watson's case, not least because the lead detective in 1932 was a very young cop who saw the case quickly spiral out of his control. Tim is having dreams about the case, and tells the retired cop who helps Det. Falsone solve it that he wondered if he had true evil (the suspect, Risley Tucker) in his sights and let him get away.

The partnership with Frank Pembleton would form a core element to the character and the entire show. Pembleton was by turns supportive and hurtful to Bayliss. He wanted to take a hard line on Bayliss's cousin who killed a Turkish exchange student, and also said that Bayliss lacked an understanding of "his dark side" so would do poorly in his job. Yet Pembleton also saved Bayliss from being charged in an incident that could have been interpreted as robbery, and once told him that he was the only person he trusted other than his wife, Mary. While Pembleton saw the world in strict black and white terms, Bayliss was far more open to accepting the shades of gray present in police work.