As competition in online crowdfunding intensifies, securing repeated funding success has become increasingly challenging. In such environments, fundraisers must adopt strategies beyond project quality to engage donors over time. One possible post-project behavior is expressing gratitude, which can signal fundraisers’ commitment and foster donor loyalty. This study examines how gratitude, conveyed through thank-you letters, influences serial crowdfunding success and donor relationship. Integrating signaling theory, social exchange theory, and crowdfunding literature, we argue that gratitude strengthens donor relationships and encourages repeated support. Analyzing data from DonorsChoose, Zillow, and NCES, we find that gratitude expression significantly enhances funding outcomes and attracts more repeat backers.