Background: Fucoidan is derived from seaweed widely used in Japanese cuisine, but little is known about its influence on glucose metabolism. To obtain information about the physiological effects of fucoidan on glucose metabolism, the digestive system, and the gustatory system controlling taste sensation in patients with type 2 diabetes, we conducted a randomized, double-blind, placebo-controlled study. Methods: Thirty patients with type 2 diabetes on diet therapy were recruited from an outpatient clinic (22 men and 8 women aged 59.10 ± 13.24 years, body mass index: 25.18 ± 3.88, hemoglobin A1c: 7.04 ± 1.24%). They were divided into 2 groups and underwent 2 interventions with a 4-week interval. One group received fucoidan for 12 weeks (a daily 60 mL test beverage containing 1,620 mg of fucoidan) and then placebo (60 mL) for the subsequent 12-week period, while the order was reversed in the other group. Evaluation was performed just before and after each intervention. Taste sensitivity was measured for 5 basic tastes by the filter paper disk method and food intake was evaluated with a validated diet questionnaire. Results: No adverse events occurred during the study period. Despite no change of the diet, stool frequency increased during fucoidan intake (from 7.78 ± 4.64/week in Week 1 to 9.15 ± 5.03/week in Week 5, P < 0.001), and it increased more in lean subjects. In 11 subjects whose stool frequency exceeded the mean value, the thresholds for sweet, salty, bitter and umami tastes were significantly reduced (enhancement of sensitivity) after fucoidan intake. In 14 subjects with normal HOMA-IR (homeostatic model assessment of insulin resistance, < 2.5), hemoglobin A1c decreased after fucoidan intake (from 6.73 ± 1.00 to 6.59 ± 1.00%, P < 0.05), as did the fasting plasma level of GLP-1 (glucagon-like peptide-1, from 6.42 ± 3.52 to 4.93 ± 1.88 pmol/L, P < 0.05). Conclusion: Sustained fucoidan intake led to alterations of gastrointestinal function, including increased stool frequency and enhanced taste sensitivity, which could contribute to better control of diabetes.