Truncating variations in the gene encoding the essential sarcomeric protein titin (TTNtv) have long been known to cause dilated cardiomyopathy (DCM) and, recently, peripartum cardiomyopathy (PPCM). How these genetic variations lead to the hypocontractility that is characteristic of DCM and PPCM has remained controversial due to lack of evidence supporting either haploinsufficient or dominant negative mechanisms. To examine how this hypocontractility arises, we analyzed a set of 184 explanted posttransplant DCM hearts, 22 of which bore heterozygous titin truncations. To determine if truncated titin was present in these hearts, we used allele specific proteomic mapping of titin, immunoblotting, and a patient specific anti-truncated titin frameshift antigen antibody directed against the shortened C-terminus of truncated titin. These three different approaches show the abundant presence of truncated titin exists at the expected molecular weight and with the expected epitopes in these hearts. These results also reveal titin haploinsufficiency in this set of hearts. Staining skinned cardiomyocyte fragments with this anti-truncated titin antibody shows that truncated titin binds to the thick filament and Z-disc under sarcomeric stretch, even at supraphysiological physiological sarcomere lengths. By treating stretched sarcomeres with a high-salt solution, we can detach truncated titin from the thick filament, whereupon it recoils to the Z-disk. These results indicate that truncated titin in this patient’s heart attaches to both the Z-disk and thick filament, transmitting strain across the I-band, under varying strain. Collectively, the presence of truncated titin integrated into the sarcomere combined with a reduction in full-length titin both positively support both haploinsufficient and dominant-negative mechanisms of truncated titin pathogenesis.