Timeline of longest spaceflights is a chronology of the longest spaceflights. Many of the first flights set records measured in hours and days, the space station missions of the 1970s and 1980s pushed this to weeks and months, and by the 1990s the record was pushed to over a year and has remained there into the 21st century.

The ISS year-long mission was an 11-month-long scientific research project aboard the International Space Station, which studied the health effects of long-term spaceflight. Astronaut Scott Kelly and cosmonaut Mikhail Kornienko spent 340 days in space, with scientists performing medical experiments. Kelly and Kornienko launched on 27 March 2015 on Soyuz TMA-16M along with Gennady Padalka. The mission encompassed Expeditions 43, 44, 45 and 46. The pair safely landed in Kazakhstan on March 2, 2016, returning aboard Soyuz TMA-18M with Sergey Volkov. The mission supported the NASA Twins study, which helps shed light on the health effects of long-duration spaceflight, which is of interest for Mars missions especially.