Filamentous algae usually form dense or sparse masses that float freely or attach to other plants, rocks, or other hard substrates. Many filamentous algae attach to <i>Kappaphycus alvarezii</i> which is cultivated by farmers in the tropics. This study explained the effect of filamentous algae attachment on the morphology and growth of <i>K. alvarezii</i>. This research was conducted in one of the cultivation locations on the coast of Tanjung Tiram, southeast Sulawesi Province, Indonesia. This study used a vertical net to cultivate <i>K. alvarezii</i> thallus samples. Vertical nets are used to protect <i>K. alvarezii</i> from fish pests and attached algae. We analyze the attachment rate of filamentous algae about the morphology and growth of <i>K. alvarezii</i> thallus within 40 days. The results showed two dominant filamentous algae species, Elachista flaccida and Chaetomorpha crassa. The density of E. flaccida occurred on day 10, with a density level reaching 12 individuals/m<sup>2</sup> and increasing on day 20 to 45 individuals/m<sup>2</sup>. Chaetomorpha crassa species also appeared at 2.24 g/m<sup>2</sup> on day 20, decreased to 0.63 g/m<sup>2</sup> on day 30, and disappeared on day 40. The torn surface thallus was seen to occur on day 2.324 mm<sup>2</sup> on day ten and decreased by 104 mm<sup>2</sup> on day 20. Attachment of filamentous algae slightly impacted the growth of <i>K. alvarezii</i> thallus, although the correlation analysis did not show a significant effect. Another correlation analysis explained no significant correlation between multiple attachments of filamentous algae and torn surface thallus of <i>K. alvarezii</i>.