Grateloupia lanceolata (Okamura) Kawaguchi is a red alga native to coastal areas of East Asia. The effect of a G. lanceolata extract on lipid accumulation and reactive oxygen species (ROS) production in 3T3-L1 cells was assessed by examining adipogenic transcription factors and ROS-regulating genes at the molecular level. An ethanol extract of G. lanceolata inhibited lipid accumulation and ROS production during adipogenesis. Treatment with the G. lanceolata extract lead to a reduction in the mRNA levels of the transcription factors, peroxisome proliferator-activated receptor-γ and CCAAT/ enhancer binding protein-α, and at the protein level for the target protein, adipocyte protein 2. ROS-producing nicotinamide adenine dinucleotide phosphate hydrogen (NADPH) oxidase 4 and NADPH-producing glucose-6-phosphate dehydrogenase mRNAs decreased following G. lanceolata extract treatment. In contrast, the mRNA level of ROS scavenging enzymes, including superoxide dismutase (SOD), glutathione peroxidase, and catalase increased in the extract-treated group. The increase in SOD1 (Cu/Zn-SOD) and 2 (Mn-SOD) proteins was correlated with their mRNA levels. Additionally, the G. lanceolata extract significantly enhanced mRNA levels of adiponectin, one of the adipokines secreted from adipocytes. Our results show that G. lanceolata extract inhibited lipid accumulation and ROS production by controlling adipogenic signals and ROS regulating genes.