This is a list of the 23 longest spacewalks, also known as an extravehicular activity or EVA. "Agency" here refers to the organization under whose auspices the EVA was conducted (so a Swiss or Japanese astronaut would be listed under NASA if they wore NASA suits and were controlled by Mission Control Houston).
For details, see lists of spacewalks from 1965–1999, 2000–2014, and 2015-present.