Abstract Body (Do not enter title and authors here): Heart failure with systolic dysfunction is characterized by insufficient contractility. Standards of care for heart failure treat symptoms, however there are currently no approved therapies to increase contractility to directly improve the ability of the heart to pump blood. Previously tested positive inotropes increased heart function through mechanisms that increased intracellular calcium. Unfortunately, these early inotropes were associated with detrimental effects and worsened outcomes and therefore are not approved for long-term use. There remains a need for an alternative mechanism to increase contractility without increasing intracellular calcium. We previously demonstrated that phosphorylation of the inhibitory subunit of the troponin complex, troponin I (TnI) at serine residue 150 (S150) increases force development in ex vivo muscle by increasing calcium sensitivity. Increasing the sensitivity of the myofilament to calcium is an alternative mechanism to increase contractility without increasing intracellular calcium. We therefore hypothesize that increasing TnI-S150 phosphorylation in vivo would improve systolic function without harmful effects. To determine the effects of TnI-S150 phosphorylation in vivo, we generated a phosphorylation-mimetic mouse with TnI-S150 mutated to aspartic acid (TnI-pS150). Structural and functional measurements derived from echocardiography and hemodynamics demonstrate that TnI-pS150 mice have increased cardiac systolic function and contractility in vivo. We confirm that the mechanism for increasing in vivo function is through increased myofilament calcium sensitivity. Detrimental effects commonly observed with the use of inotropes (e.g. hypertrophy, hypertension, severe diastolic dysfunction, increased arrythmia susceptibility, increased mortality) were not observed in TnI-pS150 mice. Additionally, we did not observe any adverse long-term detrimental effects on cardiac structure and function in aged TnI-pS150 mice. These results support the phosphorylation of TnI-S150 as a novel signaling mechanism to increase systolic function without detrimental effects and is therefore a novel target for systolic heart failure therapies.