The sarcomere is the elementary contractile unit of muscles. Adult muscle cells are large and chain thousands of sarcomeres into long periodic myofibrils that attach to the skeleton. During development, muscle cells must increase in length to maintain the mechanical connection to the growing skeleton. How muscles add new sarcomeres to facilitate muscle growth is unknown. Using live imaging and high-throughput image analysis, we have now tracked the sarcomere components during the developmental growth of Drosophila muscle and found that individual sarcomeres divide along the myofibril tension axis into daughter sarcomeres. This way, new sarcomeres can be inserted into contractile and mechanically intact myofibrils. We propose that sarcomere division is triggered by tension and local sarcomere damage originating from skeletal growth and muscle contractions. Sarcomere divisions repair damaged sarcomeres, ensure their mechanical integrity, and synchronize sarcomere addition with skeletal growth during animal development.