Hafsteinn Ægir Geirsson (born 4 August 1980 in Reykjavík) is an Icelandic cyclist and retired sailor, who specialized in the Laser class. He represented his nation Iceland in two editions of the Olympic Games (2000 and 2004), and has also been training throughout most of his sporting career for Þytur Sailing Regatta (Icelandic: Siglingaklúbburinn Þytur).

Hafsteinn made his official debut at the 2000 Summer Olympics in Sydney, where he placed forty-second in the Laser class with a net grade of 355, after having decided to pull himself out in the last race due to yachting malfunction.

At the 2004 Summer Olympics in Athens, Hafsteinn qualified for his second Icelandic team, as a 24-year-old, in the Laser class by granting a tripartite invitation from the International Sailing Federation. Sailing through the race series with a similar effort from the previous Olympics, Geirsson recorded a satisfying grade of 344 net points to upgrade his position to fortieth in a fleet of forty-two sailors.

Hafsteinn is also a cyclist. He has won several Icelandic competitions and the Faroese Kring Føroyar (Tour of Faroe Islands) which he has won four times, 2005, 2006, 2007 and 2008.