Thermoacoustic imaging is a hybrid technique that can provide functional and molecular information of deep tissue at lower cost using non-harmful radiation compared to competing methods. It primarily maps electro- magnetic absorption contrast at both optical and radio frequencies (RF) with ultrasonic spatial resolution. In standard practice, different simulation tools are combined to simulate the hybrid thermoacoustic imaging process, which requires communication between several software packages. A general purpose solver is critical for thermoacoustic simulations, as a variety of phenomenon must be modeled in order to represent the physical reality. Here we present ONELAB as a single simulation platform for numerically simulating thermoacoustic imaging, where optical or RF propagation inside the tissue is solved in the forward excitation path, and ultra- sound propagation is solved during the backward detection path. Our validation experiments using simulation platforms mimicking both optical and RF properties of biological tissue demonstrated that ONELAB can accurately model thermoacoustic imaging. The advantages of ONELAB include a completely open source software platform that does not depend on any specific/standard software to operate. ONELAB provides a finite element meshing utility as well as a general purpose finite element solver.