Flatiron or flat iron can mean several things:
- An old term for a clothes iron
- A wedge-shaped building:
- The Flatiron Building in New York City, for which the surrounding Flatiron District is named
- Flatiron Building (disambiguation), numerous other wedge-shaped buildings
- Flatiron (geomorphology), a steeply sloping wedge shaped landscape feature
- The Flatirons, rock formations near Boulder, Colorado
- Flatiron, a volcano in Wells Gray Park, British Columbia, Canada
- Flat Iron, Indiana, a small community in Vermillion County
- Flatiron (ship type), a type of British coastal trading vessel that carried coal up the River Thames
- Flat-iron gunboat, a 19th century iron gunboat typified by a single large gun fitted in the bow.
- Flat iron steak, a cut of beef
- Flatirons Community Church, a large non-denominational church in Lafayette, Colorado
- Another term for a hair iron