Truncating variants in TTN (TTNtvs) are the most common known cause of nonischemic dilated cardiomyopathy (DCM), but how TTNtvs cause disease has remained controversial. Efforts to detect truncated titin proteins in affected human DCM hearts have failed, suggesting that disease is caused by haploinsufficiency, but reduced amounts of titin protein have not yet been demonstrated. Here, we leveraged a collection of 184 explanted posttransplant DCM hearts to show, using specialized electrophoretic gels, Western blotting, allelic phasing, and unbiased proteomics, that truncated titin proteins can quantitatively be detected in human DCM hearts. The sizes of truncated proteins corresponded to that predicted by their respective TTNtvs; the truncated proteins were encoded by the TTNtv-bearing allele; and no degradation fragments from protein encoded by either allele were detectable. In parallel, full-length titin was less abundant in TTNtv⁺ than in TTNtv⁻ DCM hearts. Disease severity or need for transplantation did not correlate with TTNtv location. Transcriptomic profiling revealed few differences in splicing or allelic imbalance of the TTN transcript between TTNtv⁺ and TTNtv⁻ DCM hearts. Studies with isolated human adult cardiomyocytes revealed no defects in contractility in cells from TTNtv⁺ compared to TTNtv⁻ DCM hearts. Together, these data demonstrate the presence of truncated titin protein in human TTNtv⁺ DCM, show reduced amounts of full-length titin protein in TTNtv⁺ DCM hearts, and support combined dominant-negative and haploinsufficiency contributions to disease.