The inhibition of marine biofouling by the bromotyrosine derivative ianthelline, isolated from the Arctic marine sponge Stryphnus fortis, is described. All major stages of the fouling process are investigated. The effect of ianthelline on adhesion and growth of marine bacteria and microalgae is tested to investigate its influence on the initial microfouling process comparing with the known marine antifoulant barettin as a reference. Macrofouling is studied via barnacle (Balanus improvisus) settlement assays and blue mussel (Mytilus edulis) phenoloxidase inhibition. Ianthelline is shown to inhibit both marine micro- and macrofoulers with a pronounced effect on marine bacteria (minimum inhibitory concentration (MIC) values 0.1–10 μg/mL) and barnacle larval settlement (IC50 = 3.0 μg/mL). Moderate effects are recorded on M. edulis (IC50 = 45.2 μg/mL) and microalgae, where growth is more affected than surface adhesion. The effect of ianthelline is also investigated against human pathogenic bacteria. Ianthelline displayed low micromolar MIC values against several bacterial strains, both Gram positive and Gram negative, down to 2.5 μg/mL. In summary, the effect of ianthelline on 20 different representative marine antifouling organisms and seven human pathogenic bacterial strains is presented.