Sargassum pallidum (Turn.) C. Ag., a kind of brown algae distributed mainly in the Shandong and Liaoning Provinces of China, has been used in folk medicine for a long time in China [1]. Only several polysaccharides from S. pallidum with antitumor and antioxidant activities have been reported [2]. However, there are no reports on phytochemical analysis of the secondary metabolites. In this paper, two new natural keto-acid derivatives, 2,6,6-trimethyl-4-oxo-2-cyclohexene-1-acetic acid (1) and 2,6,6-trimethyl-4-oxo-2-cyclohexene-1-acetic acid methyl ester (2), were reported from the whole plant of S. pallidum (Turn.) C. Ag.. Their structures were established by comprehensive analysis of the spectral data, especially 2D NMR spectral results. Compound 3 was isolated for the first time from the marine enviroment. Compound 1 was obtained as a yellow oil, [ ] D –7.96 (c 9.15, MeOH). The molecular formula of 1 was established as C11H16O3 from the results of ESI-MS ([M + H] + at m/z 197) and its NMR data. The molecular formula indicated four degrees of unsaturation within the molecule. The IR spectrum showed the presence of carbonyl groups (1653, 1651 cm–1). The 13C NMR and DEPT spectra showed that 1 has 11 carbon signals, attributable to 3 methyl, 2 methylene, 2 methine, and 4 quaternary carbons, including 1 ketone carbonyl at C 202.1 and 1 carboxyl at C 180.0. The 1H NMR spectrum together with HMQC data displayed one olefinic proton signal at H 5.79 (s), one triple signal due to one methine proton at H 2.70, two methylene group signals at H 2.02, 2.40 and H 2.17, 2.49, and three methyl group signals at H 1.03 (s, CH3), 1.04 (s, CH3), 2.03 (s, CH3). The whole structure of 1 was mainly determined by the HMBC spectra (Table 1). The correlations from H-9 proton to C-1, C-2, C-6, C-11 and the correlations from H-1 to C-2, C-3, C-6, C-9, C-11 suggested that C-1 was directly connected with C-2, C-6, and C-9. The correlations from H-5 to C-1, C-3, C-4, C-6, C-7, C-8, and the correlations from H-7, H-8 to C-1, C-5, C-6 indicated that C-5 was linked with C-4 and C-6. The correlations from H-3 to C-1, C-2, C-10 and the correlations from H-5 to C-3 confirmed that there were linkages between the C-3 and C-2, C-4 positions. Thus protons of 10-CH3 showed the HMBC correlations to the C-1, C-2 and C-3, placing the methyl group at the C-2 position. Thus, the structure of compound 1 was identified as 2,6,6-trimethyl-4-oxo-2-cyclohexene-1-acetic acid. As is known, compound 1 has not been reported previously as a natural product and was only obtained as a synthetic compound [3, 4].