The L-type Ca channel Ca1.2 controls gene expression, cardiac contraction, and neuronal activity. Calmodulin (CaM) governs Ca1.2 open probability (Po) and Ca-dependent inactivation (CDI) but the mechanisms remain unclear. Here, we present electrophysiological data that identify a half Ca-saturated CaM species (Ca/CaM) with Ca bound solely at the third and fourth EF-hands (EF3 and EF4) under resting Ca concentrations (50-100 nM) that constitutively preassociates with Ca1.2 to promote Po and CDI. We also present an NMR structure of a complex between the Ca1.2 IQ motif (residues 1644-1665) and Ca/CaM, a calmodulin mutant in which Ca binding to EF1 and EF2 is completely disabled. We found that the CaM N-lobe does not interact with the IQ motif. The CaM C-lobe bound two Ca ions and formed close contacts with IQ residues I1654 and Y1657. I1654A and Y1657D mutations impaired CaM binding, CDI, and Po, as did disabling Ca binding to EF3 and EF4 in the CaM mutant when compared to WT CaM. Accordingly, a previously unappreciated Ca/CaM species promotes Ca1.2 Po and CDI, identifying Ca/CaM as an important mediator of Ca signaling.