Amid intensifying competition in the e-commerce market, online platforms widely use promotional activities to attract customers. However, many customers have recently found that some merchants participating in e-commerce platform promotions engage in deceptive practices such as “raising prices first before cutting them.” To advocate honest business, counter false promotions, and enhance customers’ shopping experience, a growing number of platforms have begun requiring merchants to provide price guarantee (PG) services in certain promotions. This means that customers can apply for a price difference refund from the merchant if the product is sold at a lower price during the PG period than the promotional price. On the one hand, restricting prices within a certain period obviously weakens the merchants’ pricing power, making it difficult for them to benefit from reducing prices immediately after promotions. On the other hand, by participating in platforms’ PG promotions, merchants can attract more price-sensitive customers and gain access to online and offline traffic support provided by the platforms. Therefore, in practice, not all merchants take part in platform promotions with PG requirements. Thus, PG increases the complexity of promotion decisions and related economic benefit analysis for both platforms and merchants. Motivated by these observations, this paper aims to address the following questions: (1) Under what conditions does a monopoly merchant decide to participate in an e-commerce platform promotion with a PG requirement? (2) If the merchant participates in the PG promotion, how should it adjust its pricing strategies during the promotion period and the PG period? Existing studies on PG in retailers’ dynamic pricing and online retail promotions initiated by e-commerce platforms have yielded fruitful results and meaningful conclusions, providing theoretical and methodological guidance for this paper. To the best of our knowledge, no previous literature has considered e-commerce platform promotion and PG simultaneously. However, many Chinese e-commerce platforms now require PG service as a prerequisite for participation in certain promotions. Therefore, how a merchant chooses its participation and pricing strategies in platform promotions with PG requirements deserves in-depth research. Given the above, this paper constructs a game-theoretic model comprising a merchant and an e-commerce platform to investigate the merchant’s participation and dynamic pricing strategies (i.e., whether the price in the PG period should be higher than, equal to, or lower than the promotional price) in the platform’s PG discount promotion. Specifically, in the main model— which assumes equal base-level demands in both periods—we first calculate the equilibrium outcomes under three scenarios: the merchant not participating in the PG promotion (N), the merchant participating without reducing the price in the PG period (PN), and the merchant participating with a price reduction in the PG period (PR). Then, we compare the equilibrium outcomes of scenarios PN and PR with those of scenario N to derive the merchant’s optimal participation strategy. Next, conditional on the merchant’s optimal decision to participate, we compare the outcomes between scenarios PN and PR to determine the merchant’s optimal pricing strategy. Finally, in the extension, we further consider the case where the base-level demand in the PG period is lower than that in the promotion period. The findings are as follows: First, the merchant should participate in the PG promotion when customers’ discount sensitivity is greater than or equal to their price sensitivity, or when discount sensitivity is lower than price sensitivity but the base-level demand is high. Second, after participating, the merchant should set the price in the PG period equal to or lower than the promotional price, depending on factors such as customers’ discount sensitivity, the PG application rate, and the base-level demand. Third, for the e-commerce platform, launching PG promotions is not always beneficial.