Ship timbers were discovered during the excavation of the Islamic site of al-Balīd recycled in the buildings of its citadel and the Great Mosque. These fragments stripped from the hulls of Indian Ocean medieval sewn vessels form the largest collection of archaeological evidence for the ships sailing during the Middle Islamic Period (10th–15th century CE). More than one third of the timbers bears traces of a bitumen mixture used as luting and coating to seal the ships planking to waterproof and protect their hulls, a practice generally associated with sewn-plank construction. This paper presents the result of the GC-MS, carbon and hydrogen isotope on chromatographic fractions, and X-Ray diffraction analyses on twenty-two samples of the luting extracted from fifteen timbers from al-Balīd. The nature of the constituents of the amalgam is determined showing that the luting consists of bitumen mixed with mineral matter, fat or oil and two triterpenic resins, dammar (Dipterocarpacea) and frankincense. The chemical signature of the bitumen indicates that it was likely sourced from two seeps in southwest Iranian: Mamatain and Ain Gir-Chersch Mehrgir-Dehluran. The origin of the bitumen might suggest that the luting was applied during the construction or repairing of Indian Ocean vessels in boatyards along the coasts of the Arabian/Persian Gulf or Southern and Eastern Arabia.