The red alga genus Laurencia (Rhodomelaceae, Ceramiales) is known to produce and contain a rich diversity of halogenated secondary metabolites. These compounds are dominated by two major biosynthetic groups: terpenes (sesquiterpenes, diterpenes, and triterpenes) and C₁₅-acetogenins. In the course of our investigation on red algae genus Laurencia from North Borneo Island, three new halogenated chamigranes, 7-aldehydelaurencenone B (1), 2-chloro-3-methoxy-α-chamigren-9-one (2), and laureborneone (3) along with five known halogenated derivatives such as [1(15)Z,2Z,4R,8S,9R]-8,15-dibromochamigra-1(15),2,11(12)-trien-9-ol (4), [1(15)E,2Z,4R,8S,9R]-8,15-dibromochamigra-1(15),2,11(12)-trien-9-ol (5), ma’ilion (6), laurencenone B (7), and 2-chloro-3-hydroxy-α-chamigren-9-one (8) were isolated from Laurencia majuscula. The structures of these compounds were determined based on spectroscopic data which consisted of infrared (FT-IR), H¹-NMR, C¹³-NMR, H¹-H¹-COSY, HSQC, HMBC, and NOESY. In addition, high-resolution electrospray ionization mass spectrometry (HRESIMS) was done to calculate their molecular formula and unsaturation index. The anticancer potentials of these compounds were tested against three cancer cell lines, and their IC₅₀ values were determined.