T2 mapping is a promising quantitative imaging technique for the detection of myocardial edema. Conventionally, T2 mapping is performed using T2-prepared single-shot 2D acquisitions, acquiring multiple slices in several breath-holds. While showing high accuracy and reproducibility, breath-holding limits achievable spatial resolution and heart coverage and can be challenging in very sick patients. Here we propose a free-breathing whole-heart 3D T2 mapping technique with high isotropic spatial resolution in a clinically feasible scan time. This is achieved by combining an accelerated T2-prepared acquisition with patch-based reconstruction and dictionary-based signal matching. Feasibility of the proposed method was investigated in a standardized T1/T2 phantom and healthy subjects.