The contributions of estradiol and testosterone to atherosclerotic lesion progression are not entirely understood. Cross-sex hormone therapy (XHT) for transgender individuals dramatically alters estrogen and testosterone levels and consequently could have widespread consequences for cardiovascular health. Yet, no preclinical research has assessed atherosclerosis risk after XHT. We examined the effects of testosterone XHT after ovariectomy on atherosclerosis plaque formation in female mice and evaluated whether adding low-dose estradiol to cross-sex testosterone treatments after ovariectomy reduced lesion formation. Six-week-old female ApoE−/− C57BL/6 mice underwent ovariectomy and began treatments with testosterone, estradiol, testosterone with low-dose estradiol, or vehicle alone until euthanized at 23 weeks of age. Atherosclerosis lesion progression was measured by Oil Red O stain and confirmed histologically. We found reduced atherosclerosis in the estradiol- and combined testosterone/estradiol–treated mice compared with those treated with testosterone or vehicle only in the whole aorta (−75%), aortic arch (−80%), and thoracic aorta (−80%). Plaque size was similarly reduced in the aortic sinus. These reductions in lesion size after combined testosterone/estradiol treatment were comparable to those obtained with estrogen alone. Testosterone/estradiol combined therapy resulted in less atherosclerosis plaque formation than either vehicle or testosterone alone after ovariectomy. Testosterone/estradiol therapy was comparable to estradiol replacement alone, whereas mice treated with testosterone only fared no better than untreated controls after ovariectomy. Adding low-dose estrogen to cross-sex testosterone therapy after oophorectomy could improve cardiovascular outcomes for transgender patients. Additionally, these results contribute to understanding of the effects of estrogen and testosterone on atherosclerosis progression.