Sarcomeric tropomodulin (Tmod) isoforms, Tmod1 and Tmod4, cap thin filament pointed ends and functionally interact with leiomodin (Lmod) isoforms, Lmod2 and Lmod3, to control myofibril organization, thin filament lengths, and actomyosin crossbridge formation in skeletal muscle fibers. Here, we show that Tmod4 is more abundant than Tmod1 on both the transcript and protein levels in a variety of muscle types, but the relative abundances of sarcomeric Tmods are muscle-specific. We then generate Tmod4−/− mice, which exhibit normal thin filament lengths, myofibril organization, and skeletal muscle contractile function due to compensatory upregulation of Tmod1, together with an Lmod isoform switch wherein Lmod3 is downregulated and Lmod2 is upregulated. However, RNAi depletion of Tmod1 from either wild-type or Tmod4−/− muscle fibers leads to thin filament elongation by ∼15%. Thus, Tmod1 per se, rather than total sarcomeric Tmod levels, controls thin filament lengths in mouse skeletal muscle, while Tmod4 appears to be dispensable for thin filament length regulation. These findings identify Tmod1 as the key direct regulator of thin filament length in skeletal muscle, in both adult muscle homeostasis and in developmentally compensated contexts.