We report the template-free pyrolysis of easily available natural seaweed, Undaria pinnatifida, as a single precursor, which results in “seaweed carbon” (SCup). Interestingly, thus-obtained SCup not only contains heteroatoms such as nitrogen and sulfur in its framework, but it also possesses a well-developed porous structure with high surface area. The heteroatoms in SCup originate from the nitrogen- and sulfur-containing ingredients in seaweed, whereas the porosity is created by removal of salts inherently present in the seaweed. These essential and fundamental properties make seaweed a prime choice as a precursor for heteroatom-containing highly porous carbon as a metal-free efficient electrocatalyst. As-synthesized SCup showed excellent electrocatalytic activity in the oxygen reduction reaction (ORR) in alkaline medium, which can be addressed in terms of the presence of the nitrogen and sulfur heteroatoms, the well-developed porosity, and the electrical conductivity in the carbon framework. The pyrolysis temperature was a key controlling parameter that determined the trade-off between heteroatom doping, surface properties, and electrical conductivity. In particular, SCup prepared at 1000 °C showed the best ORR performance. Additionally, SCup exhibited enhanced durability and methanol tolerance relative to the state of the art commercial Pt catalyst, which demonstrates that SCup is a promising alternative to costly Pt-based catalysts for the ORR.