In the world of omnichannel retail, some brands open a flagship store at online marketplaces, while others avert it. Focusing on a large e-commerce platform, we empirically study how flagship entry affects consumers, the platform, and various sellers on the platform. We find flagship entry may benefit consumers by expanding the choice set, by intensifying price competition within the entry brand, and by improving consumer perception for parts of the platform. In the meantime, flagship entry cannibalizes the sales of same-brand sellers, while other brands may gain as the buyer base expands on the platform. Counterfactual simulation suggests that flagship entry improves the gross merchandise value (GMV) of the platform but hurts existing sellers of the entry brand. On average, the effect on consumer welfare is more positive if the flagship entry is from a non-prominent brand than from a prominent brand, because consumers tend to lower their willingness to pay for individual sellers upon a prominent flagship entry. In hypothetical scenarios where flagship entry were accompanied by constraints on other same-brand sellers, the reduced competition would benefit the flagship store but hurt consumers.