The lymphangiosarcoma of Stewart-Treves (LST) syndrome is a rare tumor, yet it is a very aggressive malignancy. This is why early diagnosis is mandatory.• Dermoscopy may be a great tool in the early diagnosis and monitoring of LST.• This first dermoscopic description of the LST syndrome shows a lack of conspicuous lacunae with findings of a pink-white background, white lines and a combination of red, blue and black structureless areas containing red dots and globules. • Dermoscopy is also useful for ruling out other differentials, such as benign vascular proliferations.• Color heterogeneity and vascularization in dermoscopy might have a prognostic implication, because it may inform the clinician of a high percentage of tumor cells on histology.Stewart-Treves syndrome is a rare invasive lymphangiosarcoma linked with chronic lymphedema after mastectomy for breast cancer. Given the aggressive nature of the tumor, preventative measures and early diagnosis are important. This case is the first dermoscopic description of the lymphangiosarcoma of Stewart-Treves syndrome; it shares similar dermoscopic findings with angiosarcoma of the scalp after irradiation, with findings of a pink-white background, white lines and a combination of red, blue and black structureless areas containing red dots and globules. The lack of conspicuous lacunae make dermoscopy useful for ruling out other differentials such as benign vascular proliferations. Color heterogeneity and vascularization in dermoscopy might inform the clinician of a high percentage of tumor cells on histology.