Seasonal samplings were conducted at six sites on the northeastern coast of Baja California, Mexico, (between Campo Hawaii and Puertecitos), in the northern zone of the Gulf of California. A total of 140 taxa of marine algae were determined, of which 125 are cited for the first time for the area of study. The families with best representation are: Rhodomelaceae with 20 species, Corallinaceae with 16, Ceramiaceae with 11, Gracilariaceae with 9 and Dictyotaceae with 9. A higher diversity was found at the beginning of autumn, winter, at the end of spring, and a lower one in summer. Lower diversity was found at the sites located towards the north: Campo Hawaii with 12 species, El Machorro with 21 and El Faro de San Felipe with 44. These are places in which the beaches are broad and composed of sand and with few pebbles. A higher diversity was found at the sites located towards the south: El Coloradito with 75 species, Playa Santa Teresa with 76 and Puertecitos with 85. These localities are characterized by stable rocky substrates, with numerous intertidal pools. The most common species with regard to distribution and occurrence over time are: Cladophora prolifera, Struveopsis robusta, Dictyota flabellata, Gelidium pusillum, Gracilaria subsecundata, Prionitis abbreviata, Corallina vancouveriensis, Lithophyllum imitans, Spongites decipiens and Spyridia filamentosa . Twenty-one species are reported for the first time from the northern Gulf of California, and another three species represent new records for the Gulf: Gelidiocolax microsphaerica, Gastroclonium parvum , and Leathesia nana .