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The brown macroalgal genus <i>Lobophora</i> plays important ecological roles in many marine ecosystems. This group has received much attention over the past decade, and a considerable number of new species have been identified globally. However, our knowledge of the genus diversity and ecology along south-east Asian coasts are still limited. Given the growing body of research that uses a combination of molecular and morphological data to identify cryptic species, this study investigates the diversity of <i>Lobophora</i> in the western Gulf of Thailand using morphological and molecular data, as well as their interactions with scleractinian corals. A total of 36 <i>Lobophora</i> specimens were collected from 15 sites in the western Gulf of Thailand and used for molecular and morphological analyses. One mitochondrial (<i>cox</i>3) and two chloroplast (<i>psb</i>A and <i>rbc</i>L) genes were amplified and sequenced for molecular phylogenetic analyses. Based primarily on phylogenetic evidence, two new species were formally described, <i>L. chumphonensis</i> sp. nov. and <i>L. thailandensis</i> sp. nov. Additionally, <i>L. lamourouxii</i> was newly recorded from Thailand. Two new lineages of <i>Lobophora obscura</i> were identified, <i>L. obscura</i>12 and <i>L. obscura</i>13. Among the <i>Lobophora</i> species identified, three were found in interaction with corals, the most notable of which was the massive coral <i>Porites</i>. <i>Lobophora chumphonensis</i> sp. nov. only interacted with <i>Porites</i> by growing on bare coral skeleton between <i>Porites</i> colonies. Furthermore, <i>L. obscura</i>13 was observed under the branching coral <i>Pocillopora</i>. Our findings revealed that <i>Lobophora</i> presented both effects and absence of effects on coral. A thorough understanding of <i>Lobophora</i> diversity and ecology is essential for ongoing and future research on coral–macroalgal ecological relationships.