This study reports the total and bioaccessible concentrations of heavy metals (Mn, Fe, Cu, Zn, Cd and Pb) in the seaweed Caulerpa racemosa var. corynephora collected from local markets along the Andaman coast of Krabi Province, Thailand. Microwave-assisted acid digestion (EPA Method 3052) was used for sample preparation prior to total metal analysis. The in vitro Unified Bioaccessibility Method (UBM) was applied to assess the bioaccessibility of the metals in the seaweed samples. The total, gastric phase, and residual fraction concentration were determined by inductively coupled plasma-optical emission spectrometry (ICP-OES). The total amounts found in the seaweed samples, in ascending order were Cd < Pb < Cu < Zn < Mn < Fe with the mean concentrations of 0.89, 0.97, 17.4, 59.0, 63.4, and 450 mg/kg dry weight, respectively. High bioaccessibility percentages for Mn (71.8–85.3%) were observed alongside moderate bioaccessibility percentages for Cu (44.3–56.3%), Zn (37.7–47.4%), and Cd (41.8–46.7%), a low bioaccessibility percentage for Pb (22.3–32.0%), and a very low bioaccessibility percentage for Fe (11.5–16.5%). A quality control procedure was implemented which involved the analysis of a certified reference material (Seaweed NMIJ CRM 7405-a) for total metals and a mass-balance approach for the assessment of the in vitro bioaccessibility method.