Brian G. Marsden is an astronomer, the longtime director of the Minor Planet Center.

He specializes in celestial mechanics and astrometry, collecting data on the positions of asteroids and comets and computing their orbits, often from minimal observational information.

Marsden has helped recover once-lost comets and asteroids. Some asteroid and comet discoveries of previous decades were "lost" because not enough observational data had been obtained at the time to determine a reliable enough orbit to know where to look for re-observation at future dates. Occasionally, a newly discovered object turns out to be a rediscovery of a previously lost object, which can be determined by calculating its orbit backwards into the past and matching calculated positions with the previously recorded positions of the lost object. In the case of comets this is especially tricky because of nongravitational forces that can affect their orbits (emission of jets of gas from the comet nucleus), but Marsden has specialized in calculating such nongravitational forces. Notably, he successfully predicted the 1992 return of the once-lost periodic comet Swift-Tuttle.

He once proposed that Pluto should be cross-listed as both a planet and a minor planet and assigned the asteroid number 10000; however, this proposal was not accepted.

In 1989, Marsden was awarded the George Van Biesbroeck Prize by the American Astronomical Society in honor of his long-term service to astronomy.

The asteroid 1877 Marsden was named in his honour.