In the field of human mesenchymal stromal cell (MSC) research, quantitative real-time reverse transcription-polymerase chain reaction (qPCR) is the method of choice to study changes in gene expression patterns upon differentiation, application of stimuli, or of factors such as inhibitors or siRNAs. To reliably detect small changes, the use of a reference gene (RG) that is stably expressed under all conditions is essential. The large number of different RGs used in the field and the lack of validation of their suitability make the comparison between studies impossible. Therefore, this work aims to establish one single RG for mesodermal differentiation studies that use MSCs. Seven commonly used RGs (glyceraldehyde-3-phosphate dehydrogenase [GAPDH], ribosomal protein L13a [RPL13a], beta-actin [ACTB], tyrosine 3-monooxygenase/tryptophan 5-monooxygenase activation protein, zeta-polypeptide [YWHAZ], eukaryotic translational elongation factor 1 alpha [EF1α], β2-microglobulin [B2M], and 18S ribosomal RNA [18S]) were investigated concerning their mRNA expression stability during expansion of bone marrow-derived MSCs up to four passages as well as during their adipo-, chondro-, and osteogenenic differentiation on days 9, 16, and 22 after induction. RPL13a was validated for qPCR studies of MSCs (bone marrow- and placenta-derived) and, additionally, for primary human bone cells (HBCs) and the osteosarcoma cell line MG-63. GAPDH and ACTB, the two most frequently used RGs, showed the highest expression variance. The superior performance of RPL13a should make it the RG of choice for all MSC studies addressing mesodermal differentiation.