The development of an affordable strategy to concurrently enhance microalgal biomass and biocomponents is warranted for commercial applications. Here, we reported that a cheap and natural polyphenol, tannic acid, regulated the metabolic and physiological properties of the oleaginous microalga Phaeodactylum tricornutum to overproduce biocompounds. Tannic acid provision regulated the key metabolic pathways to enhance algal biomass and lipids in a dose-dependent manner without having a direct impact on photosynthesis. It reduced oxidative stress and reallocated carbon precursors toward lipogenesis. Lipidomic analyses showed that tannic acid provision unprecedentedly regulated the key lipogenic pathways, enhancing glyco- and neutral-lipids by 1.29- and 1.54-fold, respectively, whereas phospholipids were significantly altered. Tannic acid facilitated polyunsaturated fatty acid overproduction with a specific increment of EPA and DHA of 1.18- and 2.25-fold, respectively. Transcriptomic analysis demonstrated that tannic acid upregulated the expression of multiple genes involved in lipogenesis. Here, we dissected the potential of tannic acid, a natural and cheaper polyphenol, for simultaneously enhancing lipids and PUFAs without impairing physiological properties in P. tricornutum. The findings provide novel insights into the mechanistic roles of polyphenols as potential chemical modulators.