Fiber elongation is the key determinant of fiber quality and output in cotton (Gossypium hirsutum). Although expression profiling and functional genomics provide some data, the mechanism of fiber development is still not well understood. Here, a gene encoding a calcium sensor, GhCaM7, was isolated based on its high expression level relative to other GhCaMs in fiber cells at the fast elongation stage. The level of expression of GhCaM7 in the wild‐type and the fuzzless/lintless mutant correspond to the presence and absence, respectively, of fiber initials. Overexpressing GhCaM7 promotes early fiber elongation, whereas GhCaM7 suppression by RNAi delays fiber initiation and inhibits fiber elongation. Reactive oxygen species (ROS) play important roles in early fiber development. ROS induced by exogenous hydrogen peroxide (H₂O₂) and Ca²⁺ starvation promotes early fiber elongation. GhCaM7 overexpression fiber cells show increased ROS concentrations compared with the wild‐type, while GhCaM7 RNAi fiber cells have reduced concentrations. Furthermore, we show that H₂O₂ enhances Ca²⁺ influx into the fiber and feedback‐regulates the expression of GhCaM7. We conclude that GhCaM7, Ca²⁺ and ROS are three important regulators involved in early fiber elongation. GhCaM7 might modulate ROS production and act as a molecular link between Ca²⁺ and ROS signal pathways in early fiber development.