HR 8799 d is an extrasolar planet located approximately 129 light-years away in the constellation of Pegasus, orbiting the 6th magnitude Lambda Boötis star HR 8799.

The planet orbits at 24 AU from HR 8799 with an eccentricity greater than 0.04 and a period of 100 years.

Upon initial discovery, it was the innermost known planet in the HR 8799 system, but e, discovered later, is now known to be closer to their parent star.

[2][6][7][8] Near infrared spectroscopy from 995 to 1769 nanometers made with the Palomar Observatory show evidence of acetylene, methane, and carbon dioxide, but ammonia is not definitively detected.

[9] In 2021, the further detection of water and carbon monoxide in the planetary atmosphere was made with the Keck Planet Imager and Characterizer (KPIC).