The retention of endogenous insulin secretion in type 1 diabetes is an attractive clinical goal, which opens possibilities for long-term restoration of glucose metabolism. Mesenchymal stromal cells (MSCs) constitute, based on animal studies, a promising interventional strategy for the disease. This prospective clinical study describes the translation of this cellular intervention strategy to patients with recent-onset type 1 diabetes. Twenty adult patients with newly diagnosed type 1 diabetes were enrolled and randomized to MSC treatment or to the control group. Residual β-cell function was analyzed as C-peptide concentrations in blood in response to a mixed-meal tolerance test (MMTT) at 1-year follow-up. In contrast to the patients in the control arm, who showed loss in both C-peptide peak values and C-peptide when calculated as area under the curve during the 1st year, these responses were preserved or even increased in the MSC-treated patients. Importantly, no side effects of MSC treatment were observed. We conclude that autologous MSC treatment in new-onset type 1 diabetes constitutes a safe and promising strategy to intervene in disease progression and preserve β-cell function.