GTS Celebrity Millennium (formerly Millennium) is a Millennium-class cruise ship operated by Celebrity Cruises, a subsidiary of Royal Caribbean Group. As the lead ship of her namesake class, all of whom are powered by gas turbines, she was delivered in 2000 and is the oldest operating ship in Celebrity's fleet.

In February 1998, Royal Caribbean signed a letter of intent with Chantiers de l'Atlantique to build two new ships, with an option for two more, that would make up a new class of ships, dubbed the Millennium class. Designed to be an evolution from Celebrity's Century-class ships, these new ships were originally planned to measure 85,000 GT and have a guest capacity of approximately 1,900, and be delivered in June 2000 and January 2001, respectively.

The ship was the first new-build vessel for Celebrity following the merger between it and Royal Caribbean, and also Celebrity's first new-build not built by German shipyard Meyer Werft. She was launched on 7 November 1999 from the shipyard. She set out for her first set of sea trials on 7 April 2000 and her second set on 21 April 2000. She was initially scheduled to be delivered on 31 May 2000, but her delivery was delayed to 22 June 2000. She was christened in Southampton on 26 June 2000 by Robyn Roux, wife of French celebrity chef Michel Roux. Her first port of registry was Monrovia, Liberia.

Following her christening, Millennium sailed her maiden voyage on 1 July 2000 from Amsterdam to Baltic ports. Following an inaugural Europe season, she debuted in North America in New York in November 2000 before re-positioning in December to Port Everglades in Fort Lauderdale, Florida to sail cruises in the Caribbean through the winter. She sailed in the Mediterranean the following summer. Since 2001, the ship has sailed extensively throughout Europe, Australasia, and the Caribbean.

In May 2011, a female passenger in her sixties was declared missing when she failed to disembark at a port call in San Diego. Security cameras on the ship showed that she had jumped off the ship into the waters between Cabo San Lucas and San Diego.

In December 2012, the ship debuted in Asia after arriving at her homeport of Singapore, where the ship has been primarily deployed through the winter; in the summer, she has primarily sailed in Alaska.

Because of the COVID-19 pandemic, in February 2020, Celebrity cancelled all Asia cruises on Celebrity Millennium and re-deployed her to Los Angeles to sail complimentary cruises in March and April 2020 for first responders and military personnel. However, all sailings were cancelled after the company halted operations on 13 March 2020. The ship was berthed or anchored offshore near San Diego throughout much of the pandemic; after months of difficulty repatriating crew members, up to 480 crew members still remained on board the ship in May 2020.

In summer 2021, Celebrity Millennium joined Celebrity Edge in becoming the second ship in its fleet to re-start North American Operations, offering few sailings out of St. Maarten prior to being repositioned to Seattle were in July 23 she became the 2nd ship worldwide to resume Alaska sailings.