Common waterhemp [Amaranthus tuberculatus (Moq.) Sauer.] is an annual plant native to the Midwest United States and is considered a troublesome weed species. The economic importance of common waterhemp is attributable to rapid population increases, the appearance of herbicide resistant biotypes, and the emergence phenology during the cropping season. Because seed dormancy levels can determine seedling emergence patterns, and the requirements necessary to remove common waterhemp seed dormancy and enhance germination are not well understood, we characterized the genetic and physiological regulation of common waterhemp seed dormancy and how seed dormancy is regulated by environmental signals such as moisture and temperature. The results indicated that seed dormancy levels in common waterhemp are highly influenced by temperature. Interactions between daily average temperatures and seed moisture reduce seed dormancy, and temperature fluctuation alleviates dormancy and triggers germination. The heritability of seed dormancy in common waterhemp is high. The heritability of genes involved in environmental signal sensing and seed dormancy regulation is more difficult to determine, but the existence and importance of these genes in seed dormancy was confirmed. In addition, the existence of intra-specific seed dormancy variation in common waterhemp was demonstrated. The evidence suggests that selection for complex seed dormancy regulatory mechanisms in common waterhemp is possible and might be one of the reasons why this weed has successfully thrived in the Midwest Region.