Diterpenoids display a large and structurally diverse class of natural compounds mainly found as specialized plant metabolites. Due to their diverse biological functions they represent an essential source for various industrially relevant applications as biopharmaceuticals, nutraceuticals, and fragrances. However, commercial production utilizing their native hosts is inhibited by low abundances, limited cultivability, and challenging extraction, while the precise stereochemistry displays a particular challenge for chemical synthesis. Due to a high carbon flux through their native 2-C-methyl-D-erythritol 4-phosphate (MEP) pathway towards photosynthetically active pigments, green microalgae hold great potential as efficient and sustainable heterologous chassis for sustainable biosynthesis of plant-derived diterpenoids. In this study, innovative synthetic biology and efficient metabolic engineering strategies were systematically combined to re-direct the metabolic flux through the MEP pathway for efficient heterologous diterpenoid synthesis in C. reinhardtii. Engineering of the 1-Deoxy-D-xylulose 5-phosphate synthase (DXS) as the main rate-limiting enzyme of the MEP pathway and overexpression of diterpene synthase fusion proteins increased the production of high-value diterpenoids. Applying fully photoautotrophic high cell density cultivations demonstrate potent and sustainable production of the high-value diterpenoid sclareol up to 656 mg L-1 with a maximal productivity of 78 mg L-1 day-1 in a 2.5 L scale photobioreactor, which is comparable to sclareol titers reached by highly engineered yeast. Consequently, this work represents a breakthrough in establishing a powerful phototrophic green cell factory for the competetive use in industrial biotechnology.