Rationale: Junctional membrane complexes (JMCs) in myocytes are critical microdomains, in which excitation–contraction coupling occurs. Structural and functional disruption of JMCs underlies contractile dysfunction in failing hearts. However, the role of newly identified JMC protein SPEG (striated muscle preferentially expressed protein kinase) remains unclear. Objective: To determine the role of SPEG in healthy and failing adult hearts. Methods and Results: Proteomic analysis of immunoprecipitated JMC proteins ryanodine receptor type 2 and junctophilin-2 (JPH2) followed by mass spectrometry identified the serine–threonine kinase SPEG as the only novel binding partner for both proteins. Real-time polymerase chain reaction revealed the downregulation of SPEG mRNA levels in failing human hearts. A novel cardiac myocyte-specific Speg conditional knockout (MCM- Speg ^fl/fl ) model revealed that adult-onset SPEG deficiency results in heart failure (HF). Calcium (Ca ²⁺ ) and transverse-tubule imaging of ventricular myocytes from MCM- Speg ^fl/fl mice post HF revealed both increased sarcoplasmic reticulum Ca ²⁺ spark frequency and disrupted JMC integrity. Additional studies revealed that transverse-tubule disruption precedes the development of HF development in MCM- Speg ^fl/fl mice. Although total JPH2 levels were unaltered, JPH2 phosphorylation levels were found to be reduced in MCM- Speg ^fl/fl mice, suggesting that loss of SPEG phosphorylation of JPH2 led to transverse-tubule disruption, a precursor of HF development in SPEG-deficient mice. Conclusions: The novel JMC protein SPEG is downregulated in human failing hearts. Acute loss of SPEG in mouse hearts causes JPH2 dephosphorylation and transverse-tubule loss associated with downstream Ca ²⁺ mishandling leading to HF. Our study suggests that SPEG could be a novel target for the treatment of HF.