Citi Bike is a privately owned public bicycle sharing system in New York serving the New York City boroughs of the Bronx, Brooklyn, Manhattan, and Queens, as well as Jersey City and Hoboken, New Jersey. Named after lead sponsor Citigroup, it was operated by Motivate (formerly Alta Bicycle Share), with former Metropolitan Transportation Authority CEO Jay Walder as chief executive until September 30, 2018, when the company was acquired by Lyft. The system's bikes and stations use technology from Lyft.

First proposed in 2008 by the New York City Department of Transportation, Citi Bike's scheduled 2011 opening was delayed by Hurricane Sandy and technological problems. It officially opened in May 2013 with 332 stations and 6,000 bikes. By October 2017 annual expansions brought the totals to 706 stations and 12,000 bikes, making the service the largest bike sharing program in the United States. Further expansions for Citi Bike are planned to extend its service area across the Bronx, Brooklyn, Manhattan, and Queens, and increase the number of bikes to 40,000.

In October 2017 the system reached a total of 50 million rides and in July 2020 the system reached 100 million rides. As of 2023^[update], there are 180,000 annual subscribers. Monthly average ridership numbers increased above 100,000 for the first time in June 2021. The all-time record for ridership in a single month occurred in October 2024^[update], when the system had 5.13 million rides.

In an effort to reduce emissions, road wear, collisions, and road and transit congestion and to improve public health, the New York City Department of Transportation (NYCDOT) researched alternative forms of transportation in the 2000s, publishing a strategic plan in 2008. According to NYCDOT statistics, 56% of all automobile trips within the city were under 3 miles (4.8 km) (with 22% under 1 mile (1.6 km) and 10% under 0.5 miles (0.80 km)), well within distances readily served by bicycle. To encourage residents to use bicycles more, the city committed to expanding bike lane miles, bike racks, and bike-parking shelters. In the 2009 bicycle share feasibility report, the New York City Department of City Planning (DCP) recommended building out the system in three phases in the four most populous boroughs, but no timeline was made public.

The city, which had already been encouraging cycling as transportation, decided to establish a bicycle share program of the kind that had seen success in other cities. In 2011, it selected Alta Bicycle Share to operate the bike share in New York City. Citi Bike was created as a public–private partnership operated by NYC Bike Share LLC, a wholly owned subsidiary of Alta. The system, which was first supposed to start in fall 2011, was pushed back to summer 2012 due to uncertainties about where to place the rental stations. The city had originally intended to place Citi Bike stalls mostly on sidewalks and public plazas, but there were some locations where stalls would take up parking spaces. Only one phase of the program was to be implemented in 2012, with more phases to come later.

Software problems delayed the planned start until March 2013. These problems were also reported by Alta programs in Chicago and in Chattanooga, Tennessee. The problems reportedly occurred because the Public Bike System Company, a Canadian affiliate of Alta, was involved in a dispute with software supplier 8D Technologies. Then, Hurricane Sandy damaged 1,000 bicycles and 60 stations in storage at Brooklyn Navy Yard. As planning progressed, some residents expressed dismay at the lack of docking stations in their neighborhoods while others fought against stations on their blocks.

Citi Bike began operations on May 27, 2013, with 332 stations. The stations were located in Manhattan south of 59th Street and in Brooklyn north of Atlantic Avenue and west of Nostrand Avenue. Officials said the system opened with 6,000 bikes. At the time of implementation, it was the largest bikesharing program in the United States. When launched, the system was slated to expand to 10,000 bicycles and 600 stations in Manhattan south of 79th Street, plus stations in several Brooklyn neighborhoods, including Greenpoint, Crown Heights, Bedford-Stuyvesant, Park Slope and Carroll Gardens as well as parts of Queens. At first, the timeline for the expansion was not made publicly available, but it was later announced that the expansion would be complete by the end of 2016. There has been increasing interest in further expansion across New York City; for example, in June 2013 a Brooklyn politician opened a petition drive to accelerate deployment in Greenpoint. In contrast, the May 2013 installation deliberately bypassed South Williamsburg.

Throughout the first year, there were more than 100,000 registered members who rode over 14,700,000 miles (23,700,000 km), including 70,000 members in the first three months alone. On August 6, 2013, riders took 42,010 trips, the largest single-day total for any North American bike-sharing system. In Citi Bike's first few months, some kiosks docked too many bikes while others did not have enough, so the company started using a fleet of box trucks to carry bikes between different kiosks every day. The kiosks also had some software problems in their early months. Problems included stations that did not accept payment information; kiosks where passersby could take bikes without paying because the bikes were not locked securely; and bike docks that did not work at all, forcing riders to travel to other stations.