The firmness of the flesh fruit is a very important feature in the eating process. Peach fruit is very hard during development, but its firmness slightly decreases in the later stages of development. While there has been extensive research on changes in cell wall polysaccharides during fruit ripening, little is known about the changes that occur during growth and development. In this study, we investigated the modifications in cell wall components throughout the development and ripening of peach fruit, as well as its impact on firmness. Our findings revealed a significant positive correlation between fruit firmness and cellulose content at development stage. However, the correlation was lost during the softening process, suggesting that cellulose might be responsible for the fruit firmness during development. Members of the chitinase-like protein (CTL) group are of interest because of their possible role in plant cell wall biosynthesis. Here, two CTL homologous genes,PpCTL1andPpCTL2, were identified in peach. Spatial and temporal expression patterns ofPpCTLsrevealed thatPpCTL1exhibited high expression abundance in the fruit and followed a similar trend to cellulose during fruit growth. Furthermore, silencingPpCTL1expression resulted in reduced cellulose content at 5 DAI (days after injection), this change that would have a negative effect on fruit firmness. Our results indicate thatPpCTL1plays an important role in cellulose biosynthesis and the maintenance of peach firmness during development.