Large quantities of marine algae are annually produced, and have been disposed or burned as solid waste. In this work, porous carbons were prepared from three kinds of marine algae (Enteromorpha, Laminaria, and Chlorella) by a two-step activation process. The as-prepared carbon materials were doped with cobalt (Co) and applied as catalysts for oxygen reduction reaction (ORR). Our results demonstrated that Co-doped porous carbon prepared from Enteromorpha sp. (denoted by Co-PKEC) displayed excellent catalytic performance for ORR. Co-PKEC obtained a half-wave potential of 0.810 V (vs. RHE) and a maximum current density of 4.41 mA/cm2, which was comparable to the commercial 10% Pt/C catalyst (E1/2 = 0.815 V, Jd = 4.40 mA/cm2). In addition, Co-PKEC had excellent long-term stability and methanol resistance. The catalytic ability of Co-PKEC was evaluated in a one-chamber glucose fuel cell. The maximum power density of the fuel cell equipped with the Co-PKEC cathode was 33.53 W/m2 under ambient conditions, which was higher than that of the fuel cell with a 10% Pt/C cathode. This study not only demonstrated an easy-to-implement approach to prepare robust electrochemical catalyst from marine algal biomass, but also provided an innovative strategy for simultaneous waste remediation and value-added material production.