Cassie Newman is a fictional character from The Young and the Restless, an American soap opera on the CBS network. Portrayed by Camryn Grimes, the character was introduced on March 19, 1997, by William J. Bell as the biological daughter of Sharon Newman (Sharon Case) who had been adopted. Grace Turner (Jennifer Gareis), Sharon's best friend, tracked Cassie down in hopes of reuniting her with Sharon, but decided to keep the girl for herself. A year passed before Sharon learned Cassie was her daughter, regaining custody with her husband Nicholas Newman (Joshua Morrow), who adopted her.

In 2005, the producers decided to kill off Cassie by having her die from injuries sustained in a car accident, the episode aired on May 24, 2005. The character's death was felt in storyline for years to follow, leading to the dissolution of her parents' marriage and numerous events thereafter. Grimes continued to recur following her character's death, usually in flashbacks, dreams or as a hallucination to the mentally ill; these returns aired from June 7, 2005, until June 14, 2007, and again on May 6, 2009, and March 15, 2010. Grimes' final return as a hallucination was over the course of July 8, 2013, to October 30, 2014, during which the actress would return to The Young and the Restless as a main cast member, as Cassie's previously unknown twin sister, Mariah Copeland. In January 2020, it was reported that Grimes would be reprising the role of Cassie, appearing on February 7. She then made another one-off appearance on December 24, 2021.

Both the actress and character were well received by fans and enjoyed critical acclaim, with Grimes becoming the youngest recipient of the Daytime Emmy Award for Outstanding Younger Actress in a Drama Series in 2000, at age 10.

When she debuted, Cassie was 6 years old, having been born on January 8, 1991. She was adopted by Alice Johnson (Tamara Clatterbuck) as an infant. However, in 1994, Alice disappeared and left her in the care of her elderly mother Millie. Sharon's best friend Grace Turner (Jennifer Gareis) later tracked down Cassie and took her for herself. The Pittsburgh Post-Gazette said that Grace wanted Cassie as a "pet" despite knowing Sharon was her biological mother. As she grew up, she went from a sweet child to a "rebellious" teen who began to lie to her parents. She also wanted to hang out with older kids at school, like Daniel Romalotti (Michael Graziadei) and Lily Winters (Christel Khalil). She was grounded because of her "rebellious streak". Cassie was described as a "good girl" who wanted to "fit in" but began cutting classes, giving her parents attitude and "a whole host of other angsty teenage pleasantries."

Cassie died following a car crash on the episode that aired May 24, 2005. Grimes said that she has missed the show since leaving, and was shocked to find out that Cassie was killed off. She stated: "If it's meant to happen, it's meant to happen. Everything happens for a reason, I think." Of Cassie's last scene, she said it was "ridiculously hard [...] If you watch my death scene, where Sharon is crying over me - I'm supposed to be dead - but I'm crying." If Cassie wasn't killed off, Grimes wanted her to have a love interest, stating: "I always wanted a love interest. If I'd stayed on the show, I had hoped that was where it would go eventually.

Liberty Kontranowski of About.com labeled Cassie's death as one of the "saddest and hardest-hitting storylines" of daytime. After her death, her parents began 'Cassie's Foundation', a movement "to prevent teenage drinking and driving."

Cassie was born in 1991, when her then sixteen-year-old mother Sharon Collins (Sharon Case) was pressured to sleep with her high school boyfriend, Frank Barritt (Phil Dozois), which she did. Sharon gave the child up for adoption, as she and her mother were financially struggling, and Frank wanted nothing to do with the child. Three years later, in 1994, Sharon moves to Genoa City, where she meets and eventually marries Nicholas Newman (Joshua Morrow), son of wealthy business tycoon Victor Newman (Eric Braeden).

In 1997, Sharon and Nick's newborn son, Noah Newman (Rory Gibson) becomes ill and it is believed that he may die. As a result, Sharon's best friend Grace Turner (Josie Davis, replaced mid-storyline by Jennifer Gareis) tracks down the baby Sharon gave up for adoption, Cassie, who had been adopted by a woman named Alice Johnson (Tamara Clatterbuck). However, Alice abandoned Cassie, and her frail, elderly mother Millie ended up caring for her. Millie reluctantly allows Grace to bring Cassie to Genoa City. When Noah survives, Grace decides to keep Cassie, wanting to raise the six-year-old with her boyfriend Tony Viscardi (Nick Scotti). Sharon meets Cassie, unaware that she is her daughter. A year later, Sharon learns the truth about Cassie being her daughter. In the process, Grace attempted to run away with Cassie, but her plans are foiled. Cassie is soon adopted by Sharon and Nick. At age eleven, Cassie almost drowns in a frozen pond.