Recently, the utilization of renewable biomass instead of fossil fuels for producing fuels and chemicals is receiving much attention due to the global climate change. Among renewable biomass, marine algae are gaining importance as third generation biomass feedstocks owing to their advantages over lignocellulose. Particularly, red macroalgae have higher carbohydrate contents and simpler carbohydrate compositions than other marine algae. In red macroalgal carbphydrates, 3,6-anhydro-L-galactose (AHG) is the main sugar composing agarose along with D-galactose. However, AHG is not a common sugar and is chemically unstable. Thus, not only AHG but also red macroalgal biomass itself cannot be efficiently converted or utilized. Here, we biologically upgraded AHG to a new platform chemical, its sugar alcohol form, 3,6-anhydro-l-galactitol (AHGol), an anhydrohexitol. To accomplish this, we devised an integrated process encompassing a chemical hydrolysis process for producing agarobiose (AB) from agarose and a biological process for converting AB to AHGol using metabolically engineered Saccharomyces cerevisiae to efficiently produce AHGol from agarose with high titers and yields. AHGol was also converted to an intermediate chemical for plastics, isosorbide. To our knowledge, this is the first demonstration of upgrading a red macroalgal biomass component to a platform chemical via a new biological route, by using an engineered microorganism.