Flagellin and its N terminal 22 amino acids peptide (flg22) function as an effective elicitor, which can induce defense responses in higher plants. Marine algae and higher plants share some conserved characteristics on defense response pathways. Flg22-induced defense responses were investigated in sporophytes of Saccharina japonica in this study. The results of Evans blue dye showed that cell death in the sporophytes of S. japonica was not observed within 2 to 6 h after flg22 induction, whereas large amounts of cell death were observed from 8 to 10 h. By using TdT-mediated dUTP-biotin nick and labeling (TUNEL), fragments with 3’-OH groups of nuclear DNA were observed. The number of 3’-OH increased gradually with the induction time and spread from the induction site. The concentration of H2O2 increased rapidly in flg22-induced sporophytes of S. japonica, with a peak of about 20 μM at 3 h by using Luminol-dependent fluorescence detection method. Histological observation of radical oxygen species (ROS) production, detected by using the fluorescent dye 2’,7’- dichlorofluorescein diacetate (DCFH-DA), showed that green DCF fluorescence intensity increased gradually with the induction time, reaching the highest level at 3 h, then decreased from 3 to 5 h. The histological observation showed a consistent result with that of quantitative analysis of H2O2. According to our data, flg22 is an effective elicitor which could induce defense responses in sporophytes of S. japonica.