Crystal Kay Williams (クリスタル・ケイ・ウィリアムズ, Kurisutaru Kei Wiriamuzu; born February 26, 1986) is a Japanese-born singer, songwriter, actress and radio host.

After releasing her first single, "Eternal Memories" (1999), Crystal Kay gained fame for her third studio album, Almost Seventeen (2002), which debuted at number 2 on the Japanese Oricon charts. Almost Seventeen eventually sold over 400,000 copies and was certified platinum by the Recording Industry Association of Japan. Crystal Kay, formerly signed to Epic Records, a sub-label of Sony Music Japan for 12 years, suddenly transferred to Delicious Deli Records, a sub-label of Universal Music Japan in 2011.

As of April 2021, she has released thirteen albums. Her seventh studio album, All Yours (2007) became Crystal Kay's only number-one release in her career when it debuted on top of the Oricon chart in June 2007. Throughout her career, Crystal Kay has collaborated with M-Flo and BoA, Chemistry as well as other well-known recording artists. With over 5 million records sold to date, Crystal Kay is one of the best-selling musicians in Japan.^{[citation needed]}

Crystal Kay was born and raised in Yokohama in Kanagawa Prefecture to a South Korean mother and an African-American father. Her father was a bassist from New Jersey and a sailor in the United States Navy stationed in Yokosuka, Japan. Her mother was a 23-year-old professional singer with one album while her father played music as a hobby. As a child, Crystal Kay met artists such as Diana Ross and Bobby Brown through her parents taking her to shows.

Crystal Kay attended Nile C. Kinnick High School at the Yokosuka military base, and Sophia University. She briefly studied the Korean language in Seoul, learning how to read and write hangul, but does not speak Korean fluently and considers herself to be Japanese. Crystal Kay is fluent in English and Japanese, the latter influencing and making a regular appearance in her songs. She has also studied French. Crystal Kay has cited Janet Jackson, Brandy, Aaliyah and Michael Jackson as her biggest influences. In Japan she cites the acts of producer Tetsuya Komuro as her J-pop influences, particularly Speed and Namie Amuro.

Crystal Kay began singing for TV commercials as child from the age of 4. At the age of 11 years old, Crystal Kay signed to the label Epic Records in Japan when a jingle she recorded for TV CM for Vitamin water garnered attention. One year later the jingle was expanded into her debut single "Eternal Memories", on July 1, 1999. Despite peaking at number 47, "Eternal Memories" has since become Crystal Kay's eighth best-selling single. 1999 saw the release of two more singles, "Teenage Universe (Chewing Gum Baby)" on September 8 and "Komichi no Hana" on November 3, the former peaking at number 47 and the latter at number 80.

Crystal Kay's first studio album, C.L.L Crystal Lover Light was released on March 23, 2000. This album is noted by fans to have a more indie and acoustic feel as opposed to her later albums which mainly come under the genres of pop and R&B. Crystal Kay was only 13 years old when this album was released.

C.L.L Crystal Lover Light debuted at number 60 on the Oricon charts. A fourth single was released on the same day as C.L.L Crystal Lover Light, "Shadows of Desire". Because of its release date it became Crystal Kay's first single to fail to chart. "Shadows of Desire" was Crystal Kay's first A-side to be sung entirely in English.