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Ancho-type pepper (<i>Capsicum annuum</i> L.) is a crop susceptible to <i>Pythium ultimum</i>, which has already been controlled with synthetic fungicide applications; however, marine antagonist microorganisms could be an alternative source of control. The efficiency in vitro and in vivo of marine bacteria and yeasts was determined against <i>P. ultimum</i>. The inhibition of the radial growth of <i>P. ultimum</i> was quantified in vitro by the bacteria <i>Stenotrophomonas rhizophila</i> KM01 and KM02; <i>Bacillus subtilis</i> RBM01 and RBM02, <i>B. amyloliquefaciens</i> 2RLBF and 3R4CF; and <i>Pseudomonas</i> spp. 2R6BF and 2RE9CF, as well as the yeasts <i>Debaryomyces hansenii</i> 1R11AB, 1R11CB, and LL01 and <i>Cryptococcus laurentii</i> 2R3BF and 2R1CB. The &#946;-1,3-glucanase activity of the marine microorganisms was quantified in the presence of the phytopathogen. The disease index (DI), growth parameters, and colony forming units (CFU) were determined in ancho-type pepper plants inoculated with marine bacteria, yeasts, and <i>P. ultimum</i>. The radial zone of the phytopathogen was inhibited by 80% and 75% by <i>S. rhizophila</i> KM01 and <i>C. laurentii</i> 2R1CB, respectively. <i>D. hansenii</i> LL01 and <i>S. rhizophila</i> KM02 showed the highest activity of &#946;-1,3-glucanase, with 6060 U/mL and 47 U/mL, respectively. <i>B. subtilis</i> RBM02 protected 100% of the plants from the oomycete, and an increase was quantified in all the growth parameters and CFU. The use of these marine bacteria and yeasts are, therefore, an option for <i>P. ultimum</i> biocontrol in ancho-type pepper plants, thereby minimizing the application of synthetic fungicides.