Phlorotannins, the major constituent in brown algae, possess various biological activities; however, there is little information their toxicological effects. To assess the safety of phlorotannins, we investigated the acute oral toxicity of a high-purity phlorotannin preparation (PRT; total phlorotannin content 90%) in beagle dogs. Six beagle dogs (3 males, 3 females) were assigned randomly to three experimental groups. PRT was administered at oral doses of 250, 500, and 750 mg/kg by capsule. Vomiting by male and female beagles was observed with 500 and 750 mg/kg on the first day. In addition, one beagle given 750 mg/kg had soft stool and diarrhea on days 3 and 13. No deaths or abnormal clinical signs were observed during the experiment. All groups showed similar body weight gain and food consumption. Our acute toxicity study showed that PRT did not cause any toxicological effects in beagle dogs.