To discover novel peroxisome proliferator-activated receptor gamma (PPARgamma) agonists that could be used as antidiabetic agents, we designed docosahexaenoic acid (DHA) derivatives (2 and 3), which have a hydrophilic substituent at the C4-position, based on the crystal structure of the ligand-binding pocket of PPARgamma. These compounds were synthesized via iodolactone as a key intermediate. We found that both DHA derivatives (2 and 3) showed PPARgamma transactivation higher than, or comparable to, that of pioglitazone, which is a TZD derivative used as an antidiabetic agent. DHA derivatives related to these potent compounds 2 and 3 were also synthesized to study structure-activity relationships. Furthermore, 4-OH DHA 2, which shows strong PPARgamma transcriptional activity, was separated as an optically pure form.